ELECTRO-ANALYSIS 


SMITH 


ELECTRO-ANALYSIS 


BY 

EDGAR   F.   SMITH 

M 
PROFESSOR    OF    CHEMISTRY,    UNIVERSITY   OF    PENNSYLVANIA 


FOURTH  EDITION,   REVISED  AND   ENLARGED 

WITH  FORTY-TWO   ILLUSTRATIONS 


OF   T 

UNIVEF 

'F.- 


PHILADELPHIA 

P.   BLAKISTON'S   SON   &   CO. 

1012    WALNUT    STREET 
1907 


COPYRIGHT,   1907,  BY  P.  BLAKISTON'S  SON  &  Co. 


PREFACE  TO   FOURTH    EDITION. 


It  appeared  advisable  to  omit  from  this  edition  the  sev- 
eral sections  relating  to  the  various  sources  of  the  current, 
particularly  those  in  which  the  older  forms  of  battery  were 
described.  It  is  true  that  the  use  of  these  sources  of  elec- 
tric energy  will  probably  continue,  but  their  construction, 
treatment  and  efficiency  are  so  well  understood  that  any 
particular  information  about  them  is  best  obtained  from 
publications  devoted  especially  to  them. 

The  greater  portion  of  the  new  material,  presented  in 
the  pages  which  follow,  refers  to  the  rapid  precipitation 
and  separation  of  metals,  the  use  of  a  mercury  cathode 
with  rotating  anode  and  the  employment  of  a  new  cell  in 
the  determination  of  cations  and  anions.  To  give  this 
material  the  space  it  so  abundantly  deserves  suggested  the 
elimination  of  the  minute  directions  found  in  the  various 
electrolytes  used  with  stationary  electrodes,  "but  it  devel- 
oped that  beginners  in  electro-analysis  learn  much  from 
the  execution  of  details,  the  handling  of  deposits  and  other 
points  which  arise  constantly  in  work  of  this  character. 
Further,  there  will  always  be  persons  who,  from  prefer- 
ence or  from  the  lack  of  facilities  to  carry  out  the  newer 
methods,  will  make  determinations  and  separations  with 
stationary  electrodes.  Indeed,  these  earlier  methods  con- 
stitute a  fundamental  step  in  the  development  of  analysis 
through  the  agency  of  the  current,  and  are  therefore  re- 
tained in  their  original  forms,  except  where  experience  has 
recommended  alterations.  So  long  as  the  time  factor  con- 

174618* 


VI  PREFACE 

tinues  to  be  of  no  moment  the  older  procedures  will  appeal 
to  the  analyst. 

It  may  be  stated  that  the  rapid  methods  of  analysis  set 
forth  in  detail  in  this  text,  including  those  in  which  the 
mercury  cathode  plays  an  important  role,  have  been  sub- 
jected to  rigorous  tests  in  this  laboratory  and  have  invari- 
ably brought  success  to  all  working  with  ordinary  care. 

The  section  describing  the  determination  of  cations  and 
anions  cannot  fail  to  excite  interest  and  inquiry.  That 
the  estimation,  for  example,  of  barium  and  chlorine,  in 
barium  chloride,  may  be  made  in  an  hour  or  less,  while 
hours  would  be  required  by  time-honored  methods,  will 
naturally  lead  one  to  pause.  The  neatness  and  accuracy 
of  such  determinations  also  recommend  them.  The  deter- 
mination of  the  ferro-  and  ferri-cyanogen  and  other  anions 
indicates  still  greater  possibilities  in  the  application  of  the 
current  to  analysis. 

The  very  latest  proposals  regarding  the  value  of  graded 
potential  in  separations  and  the  possibility  of  effecting 
organic  combustions  by  means  of  the  electric  current  re- 
ceive ample  consideration. 

The  paragraphs  on  theoretical  considerations  will  throw 
much  light  upon  the  deportment  of  metals  in  solution  and 
assist  in  explaining  many  heretofore  obscure  reactions. 

Confident  that  the  latest  advances  in  electro-chemistry 
will  win  many  additional  friends  to  this  most  interesting 
field  of  investigation,  these  prefatory  observations  may  be 
concluded  with  an  acknowledgment  of  great  indebtedness 
and  profound  gratitude  to  the  many  students  and  friends 
who  have  shared  in  this  particular  study  and  made  thereby 
possible  the  appearance  of  the  present  volume. 

S. 

THE  JOHN  HARRISON 
LABORATORY  OF  CHEMISTRY,  1907. 


TABLE   OF  CONTENTS. 


INTRODUCTION I 

SOURCES   OF   ELECTRIC   CURRENT — Magneto-Electric 

Machines,  Dynamos,  Thermopile,  Storage  Cells.  2-5 
REDUCTION  OF  THE  CURRENT — Rheostats,  Resistance 

Frame    ,  5-9 

MEASURING    CURRENTS — Voltameter,    Amperemeter., 

An  Electro-chemical  Laboratory   9~J9 

HISTORICAL  SKETCH 19~32 

THEORETICAL  CONSIDERATIONS 32~4l 

RAPID  PRECIPITATION  OF  METALS  IN  THE  ELECTRO- 
LYTIC WAY 4!~55 

USE  OF  MERCURY  CATHODE 55-63 

SPECIAL  PART. 

1.  DETERMINATION  OF  METALS   63-181 

2.  SEPARATION  OF  METALS 181-274 

3.  ADDITIONAL  REMARKS  ON  METAL  SEPARATIONS.  .  274-285 

4.  DETERMINATION  OF  THE  HALOGENS  IN  THE  ELEC- 

TROLYTIC WAY    285-289 

5.  DETERMINATION  OF  NITRIC  ACID  IN  THE  ELECTRO- 

LYTIC WAY 289-296 

6.  SPECIAL  APPLICATION  OF  THE  ROTATING  ANODE 

AND  MERCURY  CATHODE  IN  ANALYSIS 296-314 

7.  OXIDATIONS  BY  MEANS  OF  THE  ELECTRIC  CURRENT  314-319 

8.  THE  COMBUSTION  OF  ORGANIC  COMPOUNDS 3I9~33° 

INDEX 331-336 


vii 


ABBREVIATIONS. 


AM.  CH —  The  American  Chemist. 

AM.    CH.   JR =  American   Chemical  Journal. 

AM.  JR.   Sc.  AND  AR.  —  American  Journal  of  Science  and  Arts. 

AM.    PHIL.    Soc.    PR.  =  Proceedings  of  the  American  Philosophical  Society. 

ANN =  Annalen  der  Chemie  und  Pharmacie. 

BER =  Berichte    der    deutschen    chemischen    Gesellschaft. 

BERG-HUTT.    Z =  Berg-  und  Hiittenmdnnische  Zeitung. 

B.  s.  CH.  PARIS   . .  . .  —  Bulletin  de  la  Societe  Chimique  de  Paris. 
CH.  N =  Chemical  News. 

CH.    Z =  Chemiker-Zeitung. 

C.  R =  Comptes  Rendus. 

DING.  P.  JR —  Dingier  s  Polytechnisches  Journal. 

ELEKTROCH.    Z =  Elektrochemische   Zeitschrift. 

G.    CH.    ITAL —  Gazetta  chimica  italiana. 

JAHRB =  Jahresbericht  der  Chemie. 

J.    AM.    CH.    S —  Journal  of  the  American  Chemical  Society. 

JR.    AN.    CH —  Journal  of  Analytical  and  Applied  Chemistry. 

JR.  F.  PKT.  CH =  Journal  fur  praktische  Chemie. 

JR.    FR.    INS =  Journal  of  the  Franklin  Institute,  Phila. 

M.    F.    CH =  Monatsheft  fur  Chemie. 

PHIL.    MAG =  Philosophical  Magazine. 

WIED.   ANN =  Wiedemann's  Annalen. 

Z.  F.  A.  CH =  Zeitschrift  fur  analytische  Chemie. 

Z.  F.  ANG.  CH =  Zeitschrift  fur  angewandte  Chemie. 

Z.   F.   ANORG.   CH.    . .  =  Zeitschrift   fur   anorganische   Chemie. 

Z.    F.    ELEKTROCHEM.  =  Zeitschrift   fur   Elektrochemie. 

Z.  F.  PH.  CH =  Zeitschrift  fur  physikalische  Chemie. 


Vlll 


UNIVERSITY 

•  . 


ELECTRO-ANALYSIS. 


INTRODUCTION. 

Many  chemical  compounds  are  decomposed  when  exposed 
to  the  action  of  an  electric  current.  Such  a  decomposition 
is  called  Electrolysis.  The  substance  decomposed  is  termed 
an  electrolyte.  The  products  of  the  decomposition  are  the 
anions  and  cations,  or  those  ( i )  which  separate  at  the  anode, 
the  positive  electrode  or  pole  (-)-  P),  and  (2)  those  sepa- 
rating at  the  cathode,  the  negative  electrode  or  pole  ( —  P) 
of  the  source  of  the  electric  energy. 

This  behavior  of  compounds  has  become  of  great  service 
to  the  analyst,  inasmuch  as  it  has  enabled  him  to  effect  the 
isolation  of  metals  from  their  solutions,  and  by  carefully 
studying  the  electrolytic  behavior  of  salts  it  has  been  possible 
for  him  to  bring  about  quantitative  determinations  and 
separations. 

This  method  of  analysis — analysis  by  electrolysis — has 
been  designated  electro-chemical  analysis  or,  better,  Electro- 
analysis.  It  is  especially  inviting,  since  it  permits  of  clean, 
accurate  and  rapid  determinations  where  the  ordinary  meth- 
ods yield  unsatisfactory  results.  This  statement  will  at  once 
be  confirmed  on  recalling  the  gravimetric  methods  usually 
employed  in  the  estimation  of  copper,  mercury,  cadmium, 
bismuth,  tin,  or  almost  any  metal. 


ELECTRO-ANALYSIS. 

i.  SOURCES  OF  THE  ELECTRIC  CURRENT. 

The  electric  energy  required  for  quantitative  analysis  has 
been  variously  derived  from  batteries  of  well-known  types 
(see  Ayrton's  Practical  Electricity),  magneto-electric  ma- 
chines, dynamos  (see  Oettel's  Electrochemical  Experi- 
ments), thermopiles  (Z.  f.  a.  Ch.,  15,  334;  Z.  f.  ang.  Ch. 
(1890),  Heft  18,  548;  Electrotechnische  Zeitschrift,  u, 
187;  Z.  f.  a.  Ch.,  14,  350;  17,  205;  Ding.  p.  Jr.,  224, 
267;  Z.  f.  a.  Ch.,  18,  457;  25,  539),  and  electrical  accumu- 
lators or  storage  cells,  which  unquestionably  are  the  best 
source.  The  current  from  them  is  constant.  Cells  of  this 
kind  can  be  charged  from  primary  batteries,  or,  better,  by 
means  of  a  dynamo  or  thermopile.  In  any  community 
where  electric  lighting  is  employed  it  is  possible  to  have  the 
charging  done  at  little  expense,  and  in  factories,  where  there 
is  always  sufficient  power,  a  small  dynamo  could  easily  be 
arranged  for  this  purpose,  so  that  almost  any  number  of 
cells  could  be  kept  in  condition  for  work.  The  iron  esti- 
mations required  by  any  establishment  could  be  rapidly  and 
accurately  made  with  three  cells  of  this  type;  little  attention 
would  be  demanded  from  the  chemist.  While  storage 
cells  can  be  used  in  almost  every  description  of  electrolysis, 
there  are  a  great  many  cases  where  economy  would  suggest 
the  use  of  the  cheaper  batteries.  Consult  the  following 
literature  upon  storage  batteries  : 

Wied.  Ann.,  34  (1888),  583  ;  Proceedings  of  the  Royal  Society,  June  20, 
1889  ; -Transactions  of  Am.  Inst.  Mining  Engineers  (Electrical  Accumula- 
tors, Salom),  Feb.,  1890.  Elektrotechnische  Zeitschrift,  Jahrg.  1890; 
Heppe,  Akkumulatoren  fur  Elektrizitat,  Berlin,  1892;  Z.  f.  ang.  Ch.,, 
1892,  p.  451  ;  Ch.  Z.,  Jahrg.  17,  66;  Die  Akkumulatoren,  Elbs,  2te  Auflage, 
1896,  Leipzig;  Introduction  to  Electrochemical  Experiments,  F.  Oettel 
(translation  by  Smith),  Philadelphia,  1897;  Pfitzner,  Die  elektrischen 
Starkstrome,  Leipzig;  Dolezalek,  Theory  of  the  Lead  Accumulator. 


SOURCES    OF    THE    ELECTRIC    CURRENT.  3 

Stillwell  and  Austen  have  recently  suggested  the  use  of 
the  electric  light  current  for  the  determination  of  metals 
in  the  electrolytic  way.  That  portion  of  their  communi- 
cation, in  which  is  emlxxlied  all  that  is  essential  for  those 


H 
H 
£ 
< 

a 


desirous  of  adopting  this  method,  will  be  found  in  the  fol- 
lowing quotation :  "  The  whole  apparatus  can  be  made  from 
a  few  yards  of  insulated  copper  wire,  some  16  wooden  lamp 
sockets,  and  blackened  lamps,  say  six  5o-candle  power,  three 


4  ELECTRO-ANALYSIS. 

32-candle  power,  six  24-candle  power,  and  six  i6-candle 
power.  .  .  .  Binding  screws,  connections,  and  plugs  will 
also  be  necessary  in  addition  to  those  which  are  put  in  with 
the  electric  wires. 

:<  The  main  wires  +,  ±,  — ,  are  furnished  with  sockets 
A,  B,  C  for  the  introduction  of  safety  plugs,  which,  for  the 
small  currents  used  in  electrolytic  work,  need  not  exceed 
6  lamp  leads.  The  main  wires  terminate  in  binding  screws, 
by  which  they  are  connected  with  the  series  of  sockets  i,  2, 
3,  4,  5.  In  these  lamps  for  reducing  the  main  current  are 
placed,  and  if  only  one  determination  or  like  determinations 
are  required  to  be  made,  only  this  series  will  be  necessary 
if  ordinary  currents  are  required.  If,  however,  two  or  three 
different  determinations,  or  some  requiring  very  small  cur- 
rents, are  to  be  made,  side  currents  can  be  formed  as  around 
sockets  2  and  4,  and  the  current  brought  to  the  desired  size 
by  the  introduction  of  resistances  in  the  series  of  sockets 
E  and  F.  K  and  L  will  represent  the  proper  position  of 
the  solutions  to  be  electrolyzed  by  these  side  currents.  By 
this  arrangement  three  unlike  determinations  can  be  simul- 
taneously made,  one  in  the  main  circuit,  and  one  in  each  of 
the  side-series.  If  more  determinations  are  required,  other 
sets  of  sockets  may  be  put  up  and  potentials  be  taken  over 
other  lamps.  The  sockets  may  be  placed  on  the  wall  above 
the  desk,  the  wires  leading  down  to  the  solutions  to  be  elec- 
trolyzed." (Jr.  An.  Ch.,  6,  129.)  Any  other  arrangement 
can  be  adopted.  That  just  described  can  be  adjusted  to  the 
parallel  system. 

The  current  may  be  derived  from  an  Edison  three-wire 
system  or  from  any  other  incandescent  system. 

See  Herlant,  Bull,  de  TAssoc.  beige  des  Chim.,  18,  232. 

Hart  has  devised  a  resistance  frame  to  be  used  when  the 
electric  light  current  is  employed  for  electrolytic  purposes. 


REDUCTION    OF    THE    CURRENT.  5 

It  is  simpler  in  construction  than  that  described  in  the  pre- 
ceding paragraph.  Particulars  in  regard  to  it  can  be  ob- 
tained from  Baker  &  Adamson,  Easton,  Pa. 


2.  REDUCTION  OF  THE  CURRENT. 

It  is  often  necessary  to  reduce  strong  currents.     Persons 
acquainted  with  practical  physics  will  promptly  suggest  the 

FIG.  2. 


resistance  coils  found  in  physical  laboratories  as  suitable  for 
this  purpose.  They  are,  on  the  whole,  quite  satisfactory, 
and  have  been  thus  utilized,  although  simpler  and  more  con- 
venient current-reducers  have  made  their  appearance  from 
time  to  time.  A  few  of  these  later  appliances  may  be 
mentioned : 


0  ELECTRO-ANALYSIS. 

The  current  may  be  sent  through  a  saturated  solution 
of  zinc  sulphate,  contained  in  a  large  glass  cylinder,  about 
22  cm.  long  and  8.5  cm.  in  diameter.  In  one  experiment 
the  current  is  passed  from  a  to  b  (Fig.  2),  and  in  the  next 
from  b  to  a.  "  The  rod  b,  with  one  zinc  pole,  is  pushed 
toward  the  zinc  pole  a,  until  the  current  reaches  the  desired 

FIG.  3. 


strength."  It  is  well  to  amalgamate  the  zincs  from  time  to 
time.  We  are  indebted  for  this  piece  of  apparatus  to 
Classen,  who  has  also  described  another  simple  rheostat 
(Fig.  3)  (Ber.,  21,  359).  In  this  apparatus  the  current 
enters  at  a,  travels  the  German  silver  resistance  AT,  and 
returns  through  b  to  the  battery.  In  the  performance  of 
electrolytic  depositions  the  platinum  vessels,  serving  as  nega- 
tive electrodes,  may  be  connected  with  any  one  of  the  bind- 
ing-posts from  i  to  20.  This  makes  it  possible  for  the 
analyst  to  execute  eight  different  determinations  at  the  same 
time.  To  show  the  influence  of  this  apparatus,  a  current 
from  five  Bunsen  cells,  generating  68  c.c.  of  oxyhydrogen 


REDUCTION    OF    THE    CURRENT. 


gas  per  minute,  was  allowed  to  act  upon  copper  solutions 
contained  in  six  vessels.  The  current  at  binding-post  i 
was  found  to  be  equal  to  3.75  amperes;  at  2,  it  equaled 
2.617  amperes;  at  3,  2.085  amperes;  at  4,  1.911  amperes, 
etc.,  until  at  20  it  was  only  0.098  of  an  ampere. 

To  better  understand  these  figures  it  should  be  remem- 
bered that  an  ampere  equals  10.436  c.c.  of  oxyhydrogen  gas 
per  minute,  or  it  is  equivalent  to  a  current  which  will  pre- 
cipitate 19.69  mg.  of  metallic  copper,  or  67.1  mg.  of  metallic 
silver  in  one  minute. 

For  a  larger  form  of  apparatus  somewhat  similar  to  that 
described  above,  see  Ber.,  17,  1787.  Figs.  4  and  5  rep- 
resent other  forms  of  convenient  and  helpful  rheostats. 


FIG.  4. 


FIG.  5. 


The  writer  has  for  some  time  employed  a  much  simpler 
current-reducer,  which  has  the  advantage  of  cheapness  and 
ready  construction  to  recommend  it.  It  consists  of  a  light 
wooden  parallelogram,  about  six  feet  in  length.  Extending 
from  end  to  end,  on  both  sides,  is  a  light  iron  wire,  meas- 
uring in  all  about  500  feet  (Fig.  6).  With  the  binding- 


8 


ELECTRO-ANALYSIS. 


posts  at  a  and  b,  and  a  simple  clamp,  it  is  possible  to  throw 
in  almost  any  resistance  that  may  be  required.  It  answers 
all  practical  purposes. 


FIG.  6. 


LITERATURE. — v.  Klobukow,  Jr.  f.  pkt.  Ch.,  37,  375  ;  40,  121 ;  Oettel's 
Electrochemical   Experiments    (Smith),   P.    Blakiston's   Son   &   Co.,    Phila. 


MEASURING    CURRENTS. 


3.  MEASURING  CURRENTS,  VOLTAMETER, 
AMPEREMETER. 

In  every  analysis  by  electrolysis  it  is  advisable  that  the 
strength  of  the  acting  current  should  be  known.  The  Bun- 
sen  voltameter  may  be  used  for  this  purpose.  Voltameters 
of  this  description  are,  however,  only  in  rare  cases  adapted 
for  current  measurement  by  introduction  into  the  circuit. 
To  read  them  the  current  must  generally  be  interrupted,  and 
they  augment  the  resistance  of  the  circuit  to  a  marked 
degree,  hence  many  chemists  substitute  a  galvanometer 
(tangent  or  sine)  for  the  voltameter.  The  deflection  of  the 
needle  by  the  current  measures  the  strength  of  the  latter. 
"  In  order  to  express  in  terms  of  chemical  action  the  deflec- 
tion of  the  needle,  it  is  placed  in  the  same  current  with  a 
voltameter,  and  the  deviation  of  the  needle  is  observed,  as 
well  as  the  volume  of  electrolytic  gas  (reduced  to  o°  and 
760  mm.  pressure)  which  is  produced  in  a  minute.  Plac- 
ing the  volume  equal  to  v,  the  quotient  ^-a  gives  the 
standard  value  for  the  galvanometer.  If  this  standard 
value  is  denoted  by  R,  the  strength,  I,  of  a  current  which 
produces  the  deviation  a  is  I  =  R  tan.  a." 

The  writer  has  found  the  amperemeter  of  Kohlrausch 
very  satisfactory,  especially  in  cases  where  strong  currents 
are  employed.  In  this  instrument  the  current  travels 
through  an  insulated  wire  surrounding  a  bar  of  soft  iron. 
The  latter,  in  its  magnetized  state,  attracts  a  needle  or  indi- 
cator and  causes  it  to  move  over  a  vertical,  graduated  scale 
(in  amperes),  and. its  deflection  gives  at  once  the  strength 
of  the  current  in  amperes.  The  Weston  milliamperemeters 
and  ammeters  will  also  prove  most  valuable  in  this  connec- 
tion. 


IO  ELECTRO-ANALYSIS. 

In  electrolytic  work  of  any  kind  it  is  advisable  that  the 
apparatus  intended  to  measure  the  current  strength  should 
be  in  the  circuit  during  the  entire  decomposition,  for  it  is 
only  in  this  way  that  we  can  expect  to  effect  separations 
without  encountering  unpleasant  difficulties.  It  is  neces- 
sary to  know  just  what  energy  is  required,  and  then  so 
regulate  the  current  that  the  same  is  approximately  main- 
tained throughout  the  entire  determination. 

When  metals  were  first  determined  electrolytically  no 
attention  was  given  to  certain  very  important  factors. 
"  Strong  "  and  "  feeble  "  currents,  or  currents  from  a  two- 
cell  bichromate  battery,  or  five  large  Bunsen  cells,  etc.,  were 
indicated.  Measuring  instruments  were  seldom  used. 
Rarely  was  anything  said  of  the  size  of  the  cathode  upon 
which  the  metal  was  deposited,  or  of  the  forms  of  the  anode, 
the  degree  of  dilution  of  the  solution,  and  similar  facts. 
Confusion  naturally  arose  and  contradictory  statements  of 
one  kind  and  another  were  numerous.  But  in  this,  as  in 
all  other  questions  where  there  was  a  real  desire  to  arrive 
at  the  truth,  honest  experiment  soon  pointed  the  way  in 
which  changes  were  necessary  and  also  demonstrated  the 
conditions  to  be  observed  in  order  that  satisfactory  results 
might  be  obtained.  Probably  then,  as  at  present,  the  metal 
depositions  were  mainly  made  in  platinum  dishes,  or  upon 
cylinders  or  cones.  These  receptacles,  as  well  as  the  vari- 
ous anode  forms,  will  receive  thorough  consideration  later. 
It  is  the  purpose  of  the  writer  at  this  point  to  merely  empha- 
size the  most  essential  features  in  an  electrolytic  determina- 
tion or  separation.  Hence  note : 

i.  The  current  density.  To  this  end  the  inner  surface 
of  the  platinum  dish  in  which  the  electrolysis  is  made  should 
be  known  in  cm2 ;  its  contents,  too,  should  be  given  in  cm3 
for  various  heights.  N.D100  is  the  normal  density  of  the 


MEASURING    CURRENTS.  I  I 

current;  this  is  equivalent  to  the  current  strength  for  100 
cm2  of  the  electrode  surface.  The  density  (D)  therefore 
is  dependent  upon  the  current  strength,  as  well  as  upon  the 
surface  (E)  of  the  electrode  upon  which  the  metallic  deposit 
is  precipitated,  i.  e.,  D  — ^. 

When  the  surface  upon  which  the  metal  is  deposited 
equals  E,  the  corresponding  current  strength  can  be  deduced 
from  the  formula  C—  (N.D100)  •  £.  See,  further,  Miller 
and  Kiliani,  Lehrbuch  der  analyt.  Chemie,  4th  ed.,  pp. 
17-24. 

2.  The  potential  across  the  poles, — the  pole  pressure, — 
which  is  best  determined  by  means  of  a  Weston  voltmeter 
(p.  64).     This  is  a  very  important  factor.     A  number  of 
interesting  separations  have  been  made  by  carefully  regu- 
lating the  pressure — voltage.     See  Z.   f.  ph.  Ch.,   12,  97; 
also  p.  32. 

3.  The  form  of  the  anode — whether  a  flat  spiral,  a  disk 
of  platinum,  or  a  smaller  perforated  dish,  suspended  in  the 
electrolyte — should  also  be  observed,  as  well  as  its  distance 
from  the  cathode. 

4.  The  total  dilution  of  the  electrolyte  and  its  tempera- 
ture are  items  of  value. 

5.  The  ammeter  and  voltmeter  should  always  be  in  the 
circuit. 

Under  the  individual  metals  these  points  will  be  taken 
up  more  fully.  By  strict  adherence,  however,  to  these  car- 
dinal features  no  one  need  fear  the  outcome.  It  will  in 
every  way  be  satisfactory. 

As  the  importance  of  electro-analysis  has  become  evident, 
there  has  been  marked  improvement  in  the  various  forms 
of  apparatus  used  in  this  work,  and  increased  facilities  for 
the  same  are  noticed  on  all  sides.  In  every  well-appointed 
laboratory  provision  is  made  for  this  field  of  study,  and  in 


12 


ELECTRO-ANALYSIS. 


certain  institutions  rooms  are  set  aside  and  especially  equip- 
ped to  carry  out  such  work.  Here  at  the  University  of 
Pennsylvania,  where  electro-analysis  was  practiced  as  early 
as  1878,  with  no  special  appointments  and  with  the  most 
primitive  forms  of  apparatus,  there  has  been  a  gradual  evo- 
lution and  development  in  apparatus  and  facilities  according 
to  demands  and  with  increased  knowledge,  until  recently  an 
installation  has  been  made  for  this  as  well  as  for  other  lines 
of  work  in  electro-chemistry,  which  is  characterized  by  great 
completeness  and  such  simplicity  that  a  brief  sketch  of  the 
plant  may  be  well  introduced  here. 

AN  ELECTRO-CHEMICAL  LABORATORY. 
This  laboratory  will  accommodate  at  least  sixteen  stu- 
dents, working  continuously.     The  room  available  for  this 
purpose   (Fig.   7)    is  fifteen   feet  by  twenty-six  feet,   thus 

FIG.  7- 


ELECTRO-CHEMICAL  LABORATORY, 


MEASURING    CURRENTS.  13 

affording  each  individual   three   feet  by  twenty  inches  of 
table  space. 

Storage  cells  supply  the  energy.  Those  in  use  have  a 
capacity  of  120  ampere-hours,  with  a  normal  discharge  rate 
of  15  amperes  and  a  maximum  rate  of  30  amperes.  The 
compartments,  indicated  at  the  end  of  the  room,  contain 


BATTERY  ROOM. 

two  groups  of  twenty-four  cells  each.  They  supply  their 
respective  sides  of  the  room.  They  are  supported  on  racks 
of  four  shelves  each,  six  cells  per  shelf.  Each  shelf  is 
thoroughly  paraffined  and  a  half-inch  layer  of  ground  quartz 
is  placed  around  the  jars.  Fig.  8  shows  one  of  these  com- 
partments with  the  lead  wires  and  cut-outs  for  each  cell. 

The  switchboards  are  three  in  number,  two  of  them  each 
controlling  the  six  places  on  their  respective  sides  of  the 


ELECTRO-ANALYSIS. 


room,  and  the  third  controlling  the  four  places  in  the  centre. 
The  face  of  one  of  these  boards  is  shown  in  Fig.  9,  the 
letters  on  the  face  referring  to  the  working  tables  controlled. 

FIG.  9- 


DISTRIBUTING  BOARD. 

The  switchboard  on  the  east  side  of  the  room  consists  of 
a  slab  of  enameled  slate  twenty-four  by  thirty-four  inches, 
one  inch  thick,  and  contains,  for  each  of  the  six  outlets  to 
be  controlled,  one  circle  of  twenty-five  contact  pieces,  and 
has  two  spring  levers,  insulated  from  each  other  and  mov- 


MEASURING    CURRENTS.  15 

ing  about  a  common  centre,  sweeping  over  them.  The  con- 
tact blocks  are  numbered  consecutively  from  o  to  24  and  a 
stop  is  provided  to  prevent  the  levers  from  sweeping  past 
the  zero.  Cell  No.  i  is  connected  between  blocks  numbered 
o  and  i  in  each  of  the  six  circles,  cell  No.  2  between  blocks 
numbered  i  and  2,  and  so  on  for  the  remainder  of  the 
twenty-four  cells  in  that  group,  so  that  all  blocks  similarly 
numbered  on  the  one  board  are  connected  together,  and  but 
a  single  wire  leads  from  the  six  similarly  numbered  blocks 
to  the  junction  between  two  cells.  In  this  lead  is  provided 
the  usual  fuse.  The  circles  are  lettered  A,  B,  C,  etc.,  con- 
secutively, corresponding  with  the  letters  at  the  outlets  to 
be  controlled. 

Should  the  operator  at  the  outlet  E,  for  instance,  need 
two  cells,  he  goes  to  this  board,  and  finding  that  the  cells 
from  the  twelfth  cell  forward  are  not  being  used  in  any  of 
the  circles,  he  places  one  of  the  levers  on  contact  block  No. 
12  and  the  other  one  on  No.  14.  There  is  thus  very  little 
chance  of  doing  anything  wrong,  or  for  persons  to  inter- 
fere with  one  another,  because  there  is  no  necessity  to  use 
the  same  cells,  and  at  a  glance  one  can  observe  which  cells 
are  in  use.  Fig.  10  shows  the  electrical  connections  from 
one  of  these  distributing  boards  to  the  cells  and  outlets  on 
the  working  tables.  The  levers  themselves  are  too  narrow 
at  their  outer  ends  to  reach  across  from  one  block  to  an- 
other, to  prevent  short-circuiting  the  cells,  so  they  are  pro- 
vided with  fibre  extensions  on  each  side  to  prevent  their 
falling  between  the  blocks,  and  also  to  prevent  their  making 
contact  with  each  other.. 

The  switchboard  on  the  west  wall  is  exactly  similar  to 
the  one  just  described.  It  contains  the  circles  G,  H,  I,  K, 
L,  and  M,  while  the  third  one,  which  controls  the  four  out- 
lets on  the  centre  table,  is  only  twenty-four  inches  square, 


i6 


ELECTRO-ANALYSIS. 


but  has  twenty-six  contact  blocks  in  each  circle.  They  are 
numbered  o,  24,  25,  26,  and  so  on  to  48.  Between  the 
two  blocks  numbered  o  and  24  are  connected  the  cells  of  the 
group  on  the  east  side  of  the  room;  between  the  blocks  24 
and  25  is  connected  cell  No.  i  of  the  west  side  of  the  room, 
while  cell  No.  2  is  connected  between  blocks  numbered  25 
and  26.  This  arrangement  connects  the  two  groups  of 
cells  in  series,  and  permits  the  use  of  from  one  to  forty- 
eight  cells  at  the  centre  table  when  necessity  requires.  It 

FIG.  10. 


CONNECTIONS  TO  WORKING  TABLE. 

will,  perhaps,  have  been  noticed  that  there  is  no  provision 
made  for  connecting  cells  in  parallel,  and  this  is  not  neces- 
sary, as  the  maximum  discharge  rate  of  the  cells  exceeds 
the  greatest  estimated  current  needed  by  one  operator. 

All  brass  parts  on  the  back  of  the  board,  as  well  as  the 
bared  ends  of  the  wires,  are  thoroughly  coated  with  P.  and 
B.  paint,  while  the  brass  parts  on  the  front  are  heavily  lac- 
quered to  prevent  corrosion.  The  surface  of  the  contact 
blocks  can  easily  be  cleaned  with  fine  sandpaper. 

The  measuring  instruments,  after  some  deliberation,  were 


MEASURING    CURRENTS.  \J 

chosen  of  the  switchboard  type.  While  this  necessitated 
procuring  at  least  one-third  more  instruments,  yet  the  initial 
cost  was  considerably  lower  than  if  portable  instruments 
had  been  provided,  and  experience  with  portable  instruments 
has  shown  that  a  greater  accuracy  will  be  attained  with 
switchboard  instruments  of  a  good  form,  if  not  immediately, 
yet  surely  after  the  first  six  months  of  use. 

Each  outlet  is  provided  with  a  fused  switch,  a  voltmeter, 
two  ammeters,  a  rheostat,  and  a  terminal  board.  They  are 
connected  as  shown  in  Fig.  10.  The  positive  lead  after 
passing  through  the  variable  resistance  runs  directly  to  the 
positive  binding-post.  The  wire  coming  from  the  negative 
binding-post  runs  to  the  low-reading  ammeter  and  thence 
to  the  negative  side  of  the  switch,  while  the  negative  post 
marked  25  is  connected  to  the  same  switch  terminal,  but 
through  the  ammeter  of  large  capacity.  The  anode  of  the 
electrolytic  cell  is  therefore  always  connected  to  the  middle 
binding-post  and  the  cathode  either  to  post  i  or  25,  depend- 
ing upon  the  strength  of  current  it  is  intended  to  pass 
through  the  cell.  The  voltmeter,  being  connected  as  shown, 
measures  the  potential  differences  at  the  terminals  of  the 
cell,  except  for  the  addition  of  the  small  fall  of  potential 
through  the  ammeters. 

The  voltmeters  on  the  side  of  the  room  have  scales  rang- 
ing from  o  to  50,  and  divided  to  1-2  volts.  Those  on  the 
centre  table  range  from  o  to  120. 

The  ammeters  ranging  from  o  to  i  ampere  are  divided 
to  i-ioo,  and  those  reading  from  o  to  25  are  divided  to 
1-5  amperes.  The  three  instruments  are  mounted  side  by 
side  on  an  oak  backboard  extending  the  whole  length  of 
the  room  and  are  covered  by  an  air-tight  case  with  a  glass' 
front,  as  shown  in  Fig.  n.  The  cases  have  neither  doors 
nor  a  back,  but  are  simply  screwed  against  a  backboard  with 
3 


i8 


ELECTRO-ANALYSIS. 


a  heavy  felt  gasket,  making  the  joint.  The  wires  come  out 
through  hard  rubber  tubes  sealed  at  their  outer  ends  by 
insulating  tape. 


FIG.  ii. 


WORKING  TABLE. 

The  rheostats  are  of  the  enameled  type,  chosen  because 
of  their  being  impervious  to  fumes.  They  have  a  total 
resistance  of  172  ohms,  divided  into  51  steps  in  such  a  way 
that  their  resistances  form  a  geometrical  progression,  the 
first  step  and  the  sum  of  all  the  steps  being  chosen  in 
accordance  with  data  of  the  resistances  of  the  baths  deter- 
mined for  the  work  done  under  an  earlier  system. 

The  wires,  both  those  in  the  battery  rooms  and  those  in 
the  laboratory  proper,  are  covered  with  rubber,  and  those 
in  the  laboratory  are  further  encased  in  oak  moulding,  but 
this  rather  for  the  sake  of  appearance  than  for  protection. 
The  whole  installation,  as  well  as  the  other  fittings  of  the 


HISTORICAL.  19 

room,  has  a  very  neat  and  finished  appearance.  (Science, 
I3»  697  (1901).)  The  following  references  may  also  be 
consulted : 

Z.  f.  Elektrochem.,  8,  398,  445;  9,  496;  10,  238.  H.  Nissenson.. 
Einrichtungen  von  elektrolytischen  Laboratorien,  etc.  Verlag  von  W. 
Knapp  in  Halle  a.  S.  Elektrochemische  Zeitschrift  10,  267;  Gazzetta 
chimica  italiana,  36,  401;  Abegg,  Z.  f.  Elektrochem.,  12,  109;  Foerster, 
ibid.,  12,  183. 

Before  taking  up  the  description  of  the  details  to  be  ob- 
served in  the  electrolytic  precipitation  of  individual  metals, 
it  may  not  be  uninteresting  to  briefly  trace  the  history  of 
the  introduction  of  the  electric  current  into  chemical  analysis. 


4.  HISTORICAL. 

Although  the  early  years  of  last  century  show  consider- 
able activity  in  electrical  studies,  the  efforts  were  mainly 
directed  to  the  solution  of  the  physical  side  of  electrolysis. 
Cruikshank  (1801),  observing  the  readiness  with  which 
the  metal  copper  was  precipitated  by  the  current,  first  sug- 
gested it  as  a  possible  agent  in  the  detection  of  metals. 
Fischer  (1812)  detected  arsenic,  and  Cozzi  (1840)  the 
metals  generally  in  animal  fluids  by  this  means,  while  Gaul- 
tier  de  Claubry  (1850)  directed  his  efforts  wholly  to  the 
isolation  of  metals  from  poisons  by  depositing  the  same 
upon  plates  of  platinum.  When  the  precipitation  was  con- 
sidered finished  the  plates  wrere  removed,  carefully  washed, 
and  the  deposited  metals  brought  into  solution  with  nitric 
acid,  and  there  tested  for  and  identified  by  the  usual  course 
of  analysis.  The  current  was  evidently  very  feeble,  as  the 
time  recorded  as  necessary  for  the  deposition  varied  from 
ten  to  twelve  hours.  Gaultier  considered  this  method  reli- 
able in  all  instances,  but  especially  recommends  it  for  the 


20  ELECTRO-ANALYSIS. 

separation  of  copper  from  bread.  In  testing  for  zinc  he 
employed  a  strip  of  tin  as  anode,  but  states  that  a  platinum 
plate  will  answer  as  well. 

In  Graham-Otto's  Lehrbuch  der  Chemie  (1857)  it  is 
stated  that  the  oxygen  developed  at  the  positive  electrode 
readily  induces  the  formation  of  peroxides ;  .  .  .  that  lead 
and  manganese  peroxides  are  deposited,  from  solutions  of 
these  metals,  upon  the  positive  electrode  of  the  battery; 
.  .  .  that  the  point  of  a  platinum  wire,  when  attached  to 
the  anode  of  a  cell,  is  therefore  a  delicate  means  of  testing 
for  manganese  and  lead.  In  the  same  text  the  oxidizing 
power  of  the  anode  is  nicely  shown  by  the  following  simple 
experiment :  A  piece  of  iron,  in  connection  with  the  positive 
electrode  of  the  battery,  is  introduced  into  a  V-shaped  glass 
tube  containing  a  concentrated  solution  of  potassium  hy- 
droxide, while  a  platinum  wire  running  from  a  negative 
electrode  projects  into  the  other  limb  of  the  vessel.  In  a 
short  time  ferric  acid  appears  around  the  anode,  and  is 
recognized  by  its  color. 

C.  Despretz  (1857)  described  the  decomposition  of  cer- 
tain salts  by  means  of  the  electric  current,  and  remarked 
that,  while  operating  with  solutions  of  the  acetates  of  copper 
and  lead,  he  expected  both  metals  would  be  deposited  upon 
the  negative  pole,  and  was  much  surprised  to  find  that  the 
lead  separated  as  oxide  upon  the  anode  at  the  same  time  that 
the  copper  was  deposited  upon  the  cathode.  The  results 
were  the  same  when  experiments  were  conducted  with  the 
nitrates  and  pure  acetates.  With  manganese  no  deposition 
took  place  upon  the  negative  electrode,  but  a  black  oxide 
appeared  at  the  opposite  pole.  Potassium  antimonyl  tar- 
trate  gave  a  crystalline  metallic  deposit  of  antimony  at  the 
cathode,  and  upon  the  anode  a  yellowish-red  coating,  sup- 
posed to  be  anhydrous  antimonic  acid.  Bismuth  nitrate 


HISTORICAL.  2 1 

yielded  a  reddish-brown  deposit  at  the  positive  electrode. 
Despretz  concludes  his  paper  by  stating  that  although  the 
facts  were  few  in  number,  yet  they  were  new  in  so  far  as 
they  concerned  lead,  antimony,  and  manganese;  and,  fur- 
thermore, that  the  separation  of  copper  from  lead  by  the 
current  was  almost  perfectly  complete. 

Three  years  later  (1860)  Charles  L.  Bloxam  recom- 
mended the  process  of  Gaultier  for  the  detection  of  metals 
in  organic  mixtures,  although  it  may  not  be  improper  to 
add  that  Smee  (1851),  in  his  work  on  electrometallurgy, 
asserts  that  Morton  was  the  first  person  to  employ  the  elec- 
tric current  for  the  isolation  of  metals  from  poisonous  mix- 
tures. However  this  may  be,  the  fact  remains  that  Bloxam 
did  use  the  current  quite  extensively  for  this  purpose,  and 
while  he  claims  no  quantitative  results  for  the  method,  the 
apparatus  employed  by  him  and  his  subsequent  work  in  this 
direction  deserve  great  credit. 

To  detect  arsenic  electrolytically  Bloxam  made  use  of  a 
glass  jar,  four  cubic  inches  in  capacity,  closed  below  by 
parchment,  which  was  tightly  secured  by  means  of  a  thin 
platinum  wire.  In  the  neck  of  the  jar  was  a  large  cork, 
through  which  passed  a  glass  tube  bent  at  a  right  angle. 
This  tube  was  intended  to  serve  as  a  means  of  escape  for 
the  gases  liberated  within  the  jar.  The  platinum  wire  from 
the  negative  electrode  was  also  held  in  position  by  the  cork. 
The  portion  of  the  wire  within  the  jar  was  attached  to  a 
platinum  plate  dipping  into  the  arsenical  mixture  containing 
dilute  sulphuric  acid.  The  jar  with  its  contents  stood  in 
a  wide  beaker,  filled  with  water,  into  which  dipped  the  posi- 
tive electrode  of  the  battery.  Under  the  influence  of  the 
current,  metals  like  antimony,  copper,  mercury,  and  bismuth 
separated  upon  the  platinum  plate  of  the  negative  electrode, 
while  arsine  was  liberated  and  escaped  through  the  exit- 


22  ELECTRO-ANALYSIS. 

tube  into  some  suitable  absorbing  liquid.  To  ascertain  what 
metal  or  metals  had  separated  upon  the  cathode,  the  plate 
attached  thereto  was  removed,  after  the  interruption  of  the 
current,  and  treated  with  hot  ammonium  sulphide.  Upon 
evaporating  this  solution  an  orange-colored  spot  remained 
if  antimony  had  been  previously  present.  If  a  metallic 
deposit  continued  to  adhere  to  the  foil,  the  latter  was  acted 
upon  by  nitric  acid  to  effect  the  solution  of  the  remaining 
metals. 

J.  Nickles  (1862)  precipitated  silver  with  the  current 
obtained  from  a  zinc-copper  couple.  The  positive  electrode 
consisted  of  a  piece  of  graphite,  taken  from  a  lead  pencil, 
while  a  thin,  bright  copper  wire  constituted  the  negative 
electrode.  The  silver  separated  upon  this.  The  current 
was  very  feeble,  for  hydrogen  was  not  liberated  at  the 
cathode.  Nickles  also  suggested  the  reduction  of  large 
quantities  of  silver  from  the  solution  of  its  cyanide  by  this 
means.  To  obtain  the  silver  he  advised  using  a  cylindrical 
cathode  constructed  from  some  readily  fusible  alloy,  so  that 
after  the  reduction  was  finished  the  other  metals  might  be 
easily  melted  out  and  leave  a  silver  plate.  Copper,  lead, 
bismuth,  and  antimony  were  separated  electrolytically,  by 
Nickles,  from  textiles. 

In  1862  A.  C.  and  E.  Becquerel  resumed  their  electro- 
chemical investigations,  first  begun  some  thirty  years  pre- 
viously. Their  experiments  seem  to  have  been  aimed  chiefly 
toward  the  reduction  of  metallic  solutions  upon  a  large 
scale,  caring  not  for  the  quantitative  estimation  of  metals, 
but  seeking  rather  a  rapid  and  satisfactory  technical  isola- 
tion process. 

Wohler  (1868)  found  that  when  palladium  was  made 
the  positive  conductor  of  two  Bunsen  cells,  and  placed  in 
water  acidulated  with  sulphuric  acid,  it  immediately  became 


HISTORICAL.  23 

covered  with  alternating,  bright,  steel-like  colors.  He  re- 
garded the  coating  as  palladium  dioxide,  since  it  liberated 
chlorine  when  treated  with  hydrochloric  acid,  and  carbon 
dioxide  when  warmed  with  oxalic  acid.  Black  amorphous 
metal  separated  at  the  cathode.  Its  quantity  was  slight. 
Under  similar  conditions  lead  also  yields  the  brown  dioxide, 
and  the  same  may  be  said  of  thallium.  Osmium,  in  its 
ordinary  porous  form,  at  once  becomes  osmic  acid.  When 
caustic  alkali  is  substituted  for  the  acid,  the  liquid  rapidly 
assumes  a  deep  yellow  color,  while  a  thin  deposit  of  metal 
appears  upon  the  cathode.  Ruthenium  behaves  similarly 
when  applied  in  the  form  of  powder.  Osmium-iridium,  a 
compound  decomposed  with  difficulty  under  ordinary  cir- 
cumstances, immediately  passes  into  solution  when  brought 
in  contact  with  the  positive  electrode  of  a  battery  placed  in 
a  solution  of  sodium  hydroxide,  and  imparts  a  yellow  color 
to  the  alkaline  liquid.  A  black  deposit  of  metal  slowly 
makes  its  appearance  upon  the  negative  pole. 

The  experiments  thus  far  described  are  qualitative  in  their 
results.  The  first  notice  of  the  quantitative  estimation  of 
metals  electrolytically  was  that  of  Wolcott  Gibbs  (1864), 
when  he  published  the  results  he  had  obtained  with  copper 
and  nickel.  Luckow,  in  alluding  to  this  work  a  year  later 
(1865),  says:  "  I  take  the  liberty  to  observe  that  so  far  as 
the  determination  of  copper  is  concerned,  I  estimated  that 
metal  in  this  manner  more  than  twenty  years  ago,  and  as 
early  as  1860  employed  the  electric  current  for  the  deposi- 
tion of  copper  quantitatively  in  various  analyses."  It  was 
Luckow  who  proposed  the  name  Elektro-Metall  Analyse 
for  this  new  method  of  quantitative  analysis.  According 
to  this  writer  the  current  may  be  applied  as  follows : 

i.  To  dissolve  metals  and  alloys  in  acids  by  which  they 
would  not  be  affected  unaided  by  the  electric  current. 


24  ELECTRO-ANALYSIS. 

2.  To  detect  metals   like   manganese   and   lead    (silver, 
nickel,  cobalt)  ;  separating  them  in  the  form  of  peroxides; 
also  manganese  as  permanganic  acid. 

3.  To  separate  various  metals,  e.  g.,  copper  and  man- 
ganese, from  zinc,  iron,  cobalt  and  nickel. 

4.  To  deposit  and  estimate  metals  quantitatively,  in  acid, 
alkaline,  and  neutral  solutions. 

5.  For  various   reductions,   c.   g.,   silver   chloride,   basic 
bismuth  chloride,  and  lead  sulphate,  in  order  that  the  metals 
in  them  may  be  determined.     To  reduce  chromic  acid  to 
oxide,  e.  g.,  potassium  bichromate  acidulated  with  dilute 
sulphuric  acid. 

These  applications  embrace  nearly  all  that  has  since  been 
accomplished  by  the  aid  of  the  current.  In  the  same  article 
in  which  Luckow  calls  attention  to  the  facts  recorded  above, 
he  describes  minutely  the  method  pursued  by  him  in  the 
precipitation  of  metals.  Reference  to  these  early  experi- 
ments will  show  with  what  care  and  accuracy  every  detail 
was  worked  out.  Luckow  also  announced  "  that  all  the 
lead  contained  in  solution  was  deposited  as  peroxide  upon 
the  positive  electrode,  and  might  be  determined  from  the 
increased  weight  of  the  latter."  This  observation  was  fully 
confirmed  by  Hampe,  and  later  by  W.  C.  May. 

Wrightson  (1876)  called  attention  to  the  fact  that  if 
solutions  of  copper  were  electrolyzed  in  the  presence  of 
other  metals,  the  latter  greatly  influenced  the  separation  of 
the  former.  For  example,  with  copper  and  antimony,  the 
deposition  of  the  copper  was  always  incomplete  when  the 
antimony  equaled  one-fourth  to  two-thirds  the  quantity  of 
the  former.  Notwithstanding,  a  complete  separation  of  the 
two  metals  can  be  effected  when  the  quantity  of  the  anti- 
mony is  small.  A  somewhat  similar  behavior  was  noticed 
with  other  metals.  The  deposition  of  cadmium,  zinc,  cobalt, 
and  nickel  was  apparently  not  satisfactory. 


HISTORICAL.  25 

Lecoq  cle  Boisbaudran  (1877)  electrolyzed  the  potassium 
hydroxide  solution  of  the  metal  gallium,  using  six  Bunsen 
elements  with  20-30  c.c.  of  the  concentrated  liquid.  The 
deposited  metal  was  readily  detached  when  the  negative 
electrode  was  immersed  in  cold  water  and  bent  slightly. 

The  unpromising  behavior  of  zinc  solutions,  observed  by 
Wrightson,  was  fortunately  overcome  by  Parodi  and  Mas- 
cazzini  (1877),  who  employed  a  solution  of  the  sulphate, 
to  which  was  added  an  excess  of  ammonium  acetate.  Lead 
was  also  deposited  in  a  compact  form  from  an  alkaline  tar- 
trate  solution  of  this  metal  in  the  presence  of  an  alkaline 
acetate. 

After  Luckow's  experiments  upon  manganese,  little  at- 
tention appears  to  have  been  given  this  metal  until  Riche 
(1878)  published  his  results.  While  confirming  the  obser- 
vations of  Luckow,  he  discovered  that  manganese  was  not 
only  completely  precipitated  from  the  solution  of  its  sul- 
phate, but  also  from  that  of  the  nitrate,  thus  rendering  pos- 
sible an  electrolytic  separation  of  manganese  from  copper, 
nickel,  cobalt,  zinc,  magnesium,  the  alkaline  earth,  and  the 
alkali  metals.  Riche  recommended  that  the  deposited  diox- 
ide be  carefully  dried,  converted  by  ignition  into  the  proto- 
sesquioxide,  and  weighed  as  such.  According  to  this 
chemist  the  one-millionth  of  a  gram  of  manganese,  when 
exposed  to  the  action  of  the  current  gave  a  distinct  rose-red 
color,  perceptible  even  when  diluted  tenfold. 

In  zinc  depositions  Riche  gave  preference  to  a  solution 
of  zinc-ammonium  acetate  containing  free  acetic  acid. 

Luckow  was  the  first  to  mention  that  the  current  caused 
mercury  to  separate  in  a  metallic  form,  from  acid  solutions, 
upon  the  negative  electrode.  F.  W.  Clarke  (1878)  used  a 
mercuric  chloride  solution,  feebly  acidulated  with  sulphuric 
acid,  for  this  purpose.  The  deposition  was  made  in  a 
4 


26  ELECTRO-ANALYSIS. 

platinum  dish,  using  six  Bunsen  cells.  Mercurous  chloride 
was  at  first  precipitated,  but  it  was  gradually  reduced  to  the 
metallic  form.  J.  B.  Hannay  (1873)  had  previously  rec- 
ommended precipitating  this  metal  from  solutions  of  mer- 
curic sulphate,  but  gave  no  results. 

Clarke,  also,  gave  some  attention  to  cadmium ;  his  results, 
however,  were  not  satisfactory.  A  few  months  later  the 
writer  (1878)  succeeded  in  depositing  cadmium  completely 
and  in  a  very  compact  form  from  solutions  of  its  acetate. 
Upon  this  behavior  Yver  (1880)  based  his  separation  of 
cadmium  from  zinc.  Furthermore,  the  writer  found  (1880) 
that  the  deposition  of  cadmium  could  be  made  from  solu- 
tions of  its  sulphate,  contrary  to  an  earlier  observation  of 
Wrightson.  At  the  same  time  copper  was  completely  sepa- 
rated from  cadmium  by  electrolyzing  their  solution  in  the 
presence  of  free  nitric  acid. 

A  very  successful  determination  of  both  zinc  and  cad- 
mium was  published  by  Beilstein  and  Jawein  in  1879.  They 
employed  for  this  purpose  solutions  of  the  double  cyanides. 

Heinrich  Fresenius  and  Bergmann  (1880)  found  that 
the  electrolysis  of  nickel  and  cobalt  solutions  succeeded  best 
in  the  presence  of  an  excess  of  free  ammonia  and  ammonium 
sulphate. 

Their  experience  with  silver  demonstrated  that  the  best 
results  could  be  obtained  with  solutions  containing  free 
nitric  acid,  and  by  the  employment  of  weak  currents. 

The  writer  (.1880)  showed  that  if  uranium  acetate  solu- 
tions were  electrolyzed  the  uranium  was  completely  precipi- 
tated as  a  hydrated  protosesquioxide ;  and,  further,  that 
molybdenum  could  be  deposited  as  hydrated  sesquioxide 
from  warm  solutions  of  ammonium  molybdate  in  the  pres- 
ence of  free  ammonia.  Very  promising  indications  were 
obtained  with  salts  of  tungsten,  vanadium  and  cerium. 


HISTORICAL.  27 

In  a  more  recent  (1880)  communication  from  Luckow, 
to  whom  we  are  indebted  for  much  that  is  valuable  in  elec- 
trolysis, is  given  a  full  description  of  his  observations  in 
this  field  of  analysis,  from  which  the  following  condensed 
account  is  taken.  While  it  relates  more  particularly  to  the 
qualitative  behavior  of  various  compounds,  its  importance 
demands  careful  study. 

When  the  current  is  conducted  through  an  acid  solution 
of  potassium  chromate,  the  chromic  acid  is  reduced  to  oxide ; 
whereas,  if  the  solution  of  the  oxide  in  caustic  potash  be 
subjected  to  a  like  treatment,  potassium  chromate  is  pro- 
duced. Arsenic  and  arsenious  acid  behave  similarly.  The 
same  is  true  also  of  the  soluble  ferro-  and  fern-cyanides  and 
nitric  acid.  In  the  presence  of  sulphuric  acid  ferric  and 
uranic  oxides  are  reduced  to  lower  states  of  oxidation. 
Sulphates  result  in  the  electrolysis  of  the  alkaline  sulphites, 
hyposulphites,  and  sulphides,  and  carbonates  from  the  alka- 
line organic  salts.  In  short,  the  current  has  a  reducing 
action  in  acid  solutions,  and  the  opposite  effect  in  those  that 
are  alkaline.  In  the  electrolysis  of  solutions  of  hydrogen 
chloride,  bromide,  iodide,  cyanide,  ferro-  and  ferri-cyanide 
and  sulphide,  the  hydrogen  separates  at  the  electro-negative 
pole,  and  the  electro-negative  constituents  at  the  positive 
electrode.  Cyanogen  sustains  a  more  thorough  decomposi- 
tion, the  final  products  being  carbon  dioxide  and  ammonia. 
In  the  electrolysis  of  ferro-  and  ferri-cyanogen  Prussian 
blue  separates  at  the  positive  electrode.  In  dilute  chloride 
solutions  hypochlorous  acid  is  the  only  product,  whereas 
chlorine  is  also  present  in  concentrated  solutions.  In  alka- 
line chloride  solutions  chlorates  are  produced  as  soon  as  the 
liquid  becomes  alkaline.  In  the  iodides  and  bromides  iodine 
and  bromine  separate  at  the  positive  electrode,  while  bro- 
mates  and  iodates  are  formed  when  metals  of  the  first  two 


28  ELECTRO- ANALYSIS. 

groups  are  present.  Potassium  cyanide  is  converted  into 
potassium  and  ammonium  carbonates.  Concentrated  nitric 
acid  is  reduced  to  nitrous  acid;  however,  when  its  specific 
gravity  equals  1.2,  this  does  not  occur,  at  least  not  when  a 
feeble  current  is  used.  Dilute  nitric  acid  alone,  or  even  in 
the  presence  of  sulphuric  acid,  is  not  reduced  to  ammonia. 
(See  also  Z.  f.  anorg.  Ch.,  31,  289.)  If,  however,  dilute 
nitric  acid  be  present  in  a  copper  sulphate  solution  under- 
going electrolysis,  copper  will  separate  upon  the  negative 
electrode  and  ammonium  sulphate  will  be  formed.  Solu- 
tions of  nitrates  containing  sulphuric  acid  behave  analo- 
gously. Phosphoric  acid  sustains  no  change.  Silicic  acid 
separates  as  a  white  mass,  and  boric  acid,  in  crystals  unit- 
ing to  arborescent  groups,  at  the  positive  electrode. 

In  the  Ber.  d.  d.  chem.  Gesellschaft,  14  (1881),  1622, 
Classen  and  v.  Reiss  presented  the  first  of  a  series  of  papers 
upon  electrolytic  subjects,  which  continued  through  subse- 
quent issues  of  this  publication.  Their  early  work  was 
devoted  to  the  precipitation  of  metals  from  solutions  of 
their  double  oxalates.  They  also  elaborated  excellent  meth- 
ods for  antimony  and  tin.  Many  very  serviceable  forms  of 
apparatus,  intended  for  electrolytic  work,  were  devised  and 
described  by  them,  and  it  must  be  conceded  that  through 
the  activity  of  the  Aachen  School  electrolysis  acquired  more 
importance  in  the  eyes  of  the  chemical  public  than  it  ever 
before  possessed.  The  details  of  the  more  important  meth- 
ods proposed  by  Classen  and  his  co-laborers  will  receive  due 
mention  under  the  respective  metals. 

Quite  independently  of  Classen,  Reinhardt  and  Ihle  pro- 
posed zinc-potassium  oxalate  for  the  estimation  of  zinc  elec- 
trolytically ;  and  in  this  connection  it  may  not  be  improper 
to  mention  that  as  early  as  1879,  Parodi  and  Mascazzini 
(Gazetta  chimica  italiana,  8,  £78)  wrote  "  finally,  we  may 


HISTORICAL.  29 

add,  that  the  electrolytic  determination  of  antimony  and 
iron  in  their  derivatives  must  be  considered  an  accomplished 
fact  judging  from  the  experiments  we  have  happily  initiated 
in  this  important  subject;  namely,  that  antimony  is  fully 
precipitated  from  its  chloride  dissolved  in  basic  ammonium 
tartrate,  and  also  from  the  solutions  of  its  sulpho-salts, 
while  the  iron  is  deposited  from  a  ferric  solution  in  the  pres- 
ence of  acid  ammonium  oxalate." 

Both  of  these  suggestions  have  since  been  amplified  and 
vastly  improved  by  Classen  and  his  students. 

In  1883  Wolcott  Gibbs  "  gave  an  account  of  a  method  of 
electrolysis  for  the  separation  of  metals  from  their  solutions 
by  the  employment  of  mercury  as  negative  electrode,  the 
positive  electrode  being  a  plate  of  platinum.  Under  these 
circumstances,  and  with  a  current  of  moderate  force,  it  was 
found  possible  to  separate  iron,  cobalt,  nickel,  zinc,  cadmium, 
and  copper  so  completely  from  solutions  of  the  respective 
sulphates  that  no  trace  of  metal  could  be  detected  in  the 
liquid.  In  addition  it  was  found  that  phosphates  of  these 
metals  dissolved  in  dilute  sulphuric  acid  were  easily  resolved 
into  amalgams  and  free  acid,  and  the  advantages  of  the 
method  were  pointed  out  in  at  least  a  certain  number  of 
cases.  The  author  had  in  view  both  the  determination  of 
the  metal  by  the  increase  in  weight  of  the  mercury,  and  in 
particular  cases  of  the  molecule  combined  with  the  metal, 
either  by  direct  titration  or  by  known  gravimetric  methods." 
The  experiments  were  purely  qualitative,  such  being  in  the 
author's  opinion  sufficient -to  establish  the  correctness  of  the 
principle  involved.  "  It  is  to  be  hoped  that  the  determina- 
tion quantitatively  of  the  electro-negative  atoms  or  mole- 
cules united  with  the  metal  will  also  attract  attention,  the 
method  having  been  originally  intended  to  serve  the  double 
purpose."  This  method  is  not  applicable  in  the  case  of  anti- 
mony and  arsenic. 


3O  ELECTRO-ANALYSIS. 

Three  years  later  (1886)  Luckow  recommended  a  very 
similar  procedure  for  the  estimation  of  zinc. 

Moore  (1886)  also  published  new  data  upon  the  estima- 
tion of  iron,  cobalt,  nickel,  manganese,  etc.,  full  notice  of 
which  will  appear  under  these  metals. 

Whitfield  (1886)  suggested  an  indirect  determination 
of  the  halogens  electrolytically,  which  has  proved  useful. 

Brand  (1889)  succeeded  in  effecting  separations  by  util- 
izing solutions  of  the  pyrophosphates  of  different  metals. 

Smith  and  Frankel  (1889)  made  an  extended  study  of 
the  double  cyanides,  and  found  thereby  a  number  of  very 
convenient  methods  of  separation  heretofore  unrecorded. 
The  results  of  their  numerous  investigations  in  this  direc- 
tion are  given  in  detail  in  the  following  pages. 

Other  publications  relating  to  electrolysis  are  that  of 
Warwick  on  metallic  formates  (Z.  f.  anorg.  Ch.,  i,  285), 
that  of  Frankel  on  the  oxidation  of  metallic  arsenides  (Ch 
N.,  65,  54),  and  that  of  Vortmann  (Ber.,  24,  2749) 
upon  the  electro-deposition  of  metals  in  the  form  of  amal- 
gams, together  with  a  series  of  critical  reviews  of  electro- 
lytic methods  by  Rudorff  in  the  Z.  f.  ang.  Ch.,  1892. 

In  the  years  immediately  following  the  recording  of 
the  preceding  experiments  the  efforts  in  electro-analysis 
had  for  their  chief  purpose  the  perfecting  of  methods. 
The  absence  of  reliable  working  conditions  necessitated 
a  careful  review  of  earlier  suggestions,  with  the  result 
that  while  some  have  been  abandoned,  the  greater  num- 
ber have  been  re-enforced  and  -have  been  given  a  more 
favorable  and  extended  use.  Freudenberg  (1893)  revived 
the  idea  to  which  Kiliani  first  called  attention,  viz. :  that  by 
the  application  of  suitable  decomposition-pressures  metal 
separations  could  be  easily  executed  in  the  electrolytic  way. 
This  contribution,  published  in  the  Z.  f.  ph.  Ch.,  12,  97, 


HISTORICAL.  3 1 

and  epitomized  on  pp.  33-39,  should  be  seriously  studied 
by  all  persons  interested  in  electro-analysis.  Singularly 
enough,  the  separations  therein  indicated  had  been  previ- 
ously made  by  Smith  and  Frankel  (1889),  and  the  state- 
ment also  appears  that  by  the  use  of  the  double  cyanides  the 
field  of  separations  was  widely  extended.  (See  also  J.  Am. 
Ch.  S.,  16,93.) 

The  direct  determination  of  the  halogens  electrolytically 
has  been  worked  out  by  Vortmann,  Specketer  and  others. 

Other  contributions  have  considered  the  availability  of 
known  electro-chemical  methods  to  technical  analysis,  and 
many,  too,  have  been  almost  wholly  controversial  in  their 
character,  so  that  they  may  be  omitted  here.  The  literature 
references  to  them  appear  in  their  appropriate  places. 

The  most  recent  advances  in  electro-analysis  embrace 
the  rapid  determination  of  metals  by  agitation  of  the  electrp- 
lyte,  and  the  use  of  a  mercury  cathode.  A  complete  account 
of  the  results  achieved  by  these  means  will  appear  upon  the 
subsequent  pages. 

The  preceding  paragraphs  give  a  brief  outline  of  what 
has  been  accomplished  in  the  field  of  analysis  by  electroly- 
sis ;  for  further  information  consult  the  following : 

LITERATURE. — Jahrb.,  1850,  602  ;  C.  r.,  45,  449  ;  Jr.  f.  pkt.  Ch.,  73,  79 ; 
Chem.  Soc.  Quart.  Journ.,  13,  12;  Jahrb.,  1862,  610  ;  Ann.,  124,  131;  C.  r., 
55,  18;  Ann.,  146,  375  ;  Z.  f.  a.  Ch.,  3,  334;  Ding.  p.  Jr.  (1865),  231  ;  Z.  £.  a. 
Ch.,  8,  23  ;  n,  i,  9  ;  13,  183  ;  Am.  Jr.  Sc.  and  Ar.  (36  ser.),  6,  255  ;  Z.  £.  a. 
Ch.,  15,  297;  Ber.,  10,  1098;  Annales  de  Ch.  et  de  Phy.,  1878;  Am.  Jr.  Sc. 
and  Ar.,  16,  200;  Am.  Phil.  Soc.  Pr.,  1878;  Z.  f.  a.  Ch.,  15,  303;  Am.  Ch. 
Jr.,  2,  41  ;  Berg-Hutt.  Z.,  37,  41  ;  Z.  f.  a.  Ch.,  19,  i,  314,  324;  Am.  Ch.  Jr., 
i,  341;  B.  s.  Ch.  Paris,  34,  18;  Ber.,  12,  1446;  14,  1622,  2771;  17,  1611, 
2467,  2931;  18,  168,  1104,  1787;  19,  323;  21,  359,  2892,  2900;  Jr.  f.  pkt. 
Ch.,  24,  193;  Z.  f.  a.  Ch.,  18,  588;  22,  558;  25,  113;  Ch.  N.,  28,  581; 
53,  209  ;  Ber.,  25,  2492  ;  Z.  f.  ph.  Ch.,  12,  97  ;  Ber.,  27,  2060  ;  Z.  f.  Elektro- 
chem.,  2,  231,  253,  269;  Z.  f.  a.  Ch.  (1893),  32,  424.  And  the  following 
will  be  found  worthy  of  careful  study :  Ann.,  36,  32  ;  94,  i  ;  Z.  f.  a.  Ch., 


32  ELECTRO-ANALYSIS. 

19,  i;  Berb-Hiitt.  Z.,  42,  377;  Z.  f.  a.  Ch.,  22,  485.  Pa  week,  Elektro- 
technische  Zeitschrift  x,  243  ;  Foerster  and  M  ii  1 1  e  r ,  Z.  f.  Elektroch., 
8,  515;  Medicus,  Z.  f.  Elektroch.,  8,  696;  Z.  f.  Elektroch.,  8,  569; 
Per  kin,  Electrolytic  apparatus,  Ch.  N.,  88,  102;  J.  E.  Root,  Electro- 
chemical Analysis  and  the  Voltaic  Series,  Jr.  phys.  Chem.,  7,  428  ;  H  o  1  - 
lard,  Influence  of  the  Nature  of  the  Cathode  on  the  Quantitative  Separa- 
tion of  Metals  by  Electrolysis,  Ch.  N.,  88,  5  ;  ibid.,  89,  no  ;  87,  193. 


5.  THEORETICAL   CONSIDERATIONS. 

In  the  following  pages,  forms  of  apparatus  and  their 
arrangement  in  carrying  out  metal  determinations  will  be 
carefully  considered.  As  the  details  for  estimations  and 
separations  will  be  amply  given,  and  electrolytes  of  various 
descriptions  will  be  suggested,  a  preliminary  section  may  be 
here  introduced,  in  which  will  be  set  forth  some  of  the  views 
entertained,  at  present,  for  the  different  behavior  of  metals 
in  electrolytes  which  have  met  with  widest  use. 

It  is  due  Kiliani  (1883)  to  say  that  he  showed  by 
attention  to  differences  in  decomposition  pressure,  how 
the  separation  of  metals  could  be  readily  made  in  the  elec- 
trolytic way.  He  used  pressures  corresponding  closely  to 
the  thermal  values  of  the  salts  undergoing  electrolysis. 

Uncertainty  prevailed  as  to  whether  the  precipitation  of 
a  metal  first  began  when  a  definite  pressure  was  reached,  or 
whether  it  took  place  with  the  very  lowest  pressure  and 
gradually  advanced  to  the  maximum.  On  this  point  Kili- 
ani's  study  gave  no  decisive  answer. 

In  1891,  Le  Blanc  (Z.  f.  ph.  Ch.,  8,  299)  conclusively 
demonstrated  that  every  electrolyte,  under  normal  condi- 
tions, showed  a  decomposition-pressure  peculiar  to  it,  and 
that  this  pressure  might  be  accurately  determined. 

Freudenberg,  guickd  by  these  facts  (Z.  f.  ph.  Ch.,  12, 
97)  classified  the  metals  as  follows: 


THEORETICAL    CONSIDERATIONS.  33 

1.  Those  which,  by  proper  pressure,  cannot  be  separated 
from  aqueous  solutions :  the  alkali  metals,  the  alkaline  earth 
metals,  etc. 

2.  Those  generally  precipitated  on  the  anode  by  the  cur- 
rent in  the  form  of  peroxides :  lead,  manganese  and  thallium. 

3.  Those  deposited  in  metallic  form  upon  the  cathode. 
These  three  groups  may  be  easily  separated.     In  this  in- 
stance, electromotive  force  (pressure)  has  little  influence. 

But  Freudenberg  observed : 

"  The  third  or  last  group  may  be  separated  into  sub- 
groups, easily  separable  one  from  the  other,  the  important 
point  being  the  magnitude  of  their  discharge  potential  in 
comparison  with  that  of  hydrogen. 

"  According  to  Le  Blanc  the  decomposition  value  of  all 
acids  and  bases  reaches  its  maximum  at  1.7  volts.  This  is 
due  to  the  fact  that  at  this  point  the  ions  of  water  can  dis- 
charge themselves.  Therefore,  all  those  metals  whose  salt 
solutions  cannot  be  decomposed  till  the  pressure  exceeds  1.7 
volts,  must  have  a  greater  electric  cohesion  than  the  hydro- 
gen of  water.  Since  then,  in  electrolysis,  those  ions  will 
be  first  deprived  of  their  charge,  which  require  the  least 
expenditure  of  energy  to  accomplish  this,  the  metals  of  the 
last  group  will  not  be  precipitated  from  solutions  in  which 
the  hydrogen  ions,  in  proportion  to  the  current  density,  are 
present  in  excess.  This  end  is  reached  by  the  presence  of 
strong  acids,  e.  g.,  nitric  acid.  Weak  acids  will  not  answer, 
because  the  concentration  of  hydrogen  ions  in  them  is  too 
slight. 

"  Alkalies  and  alkali  salts  cannot  exercise  any  influence 
upon  the  precipitation  of  metals.  This  is  because  the  alkali 
metal  in  them  plays  the  role  of  a  cation  and  is  therefore 
not  to  be  considered  in  the  discharge.  The  most  important 
metals,  which  show  in  their  salt  solutions  a  more  ready 


34  ELECTRO-ANALYSIS. 

decomposability  than  the  corresponding  acids,  are  gold, 
platinum,  silver,  mercury,  copper,  bismuth,  antimony,  ar- 
senic and  tin.  As  previously  mentioned,  the  ratio  of  their 
decomposition  values  (being  independent  of  the  anion)  will 
be  the  same  in  all  cases,  if  there  is  only  present  in  the  solu- 
tions a  sufficient  number  of  metal  ions.  This  condition  is 
almost  invariably  realized;  because,  as  a  rule,  metallic  salts 
are  strongly  dissociated.  The  condition,  however,  is  not 
met  when  dealing  with  complex  salts.  And  it  is  especially 
true  in  the  case  of  the  metal  double  cyanides;  e.  g.,  potas- 
sium copper  cyanide.  Its  formula  indicates  it  to  be  the 
potassium  salt  of  hydro-cupro-cyanic  acid.  If  this  salt  were 
absolutely  complex,  then  it  could  only  contain  ions  of  CuCy4 
and  potassium.  Upon  electrolysis  CuCy4  would  pass  to  the 
anode  and  potassium  to  the  cathode.  A  precipitation  of 
copper  could  not  occur.  As  a  matter  of  fact,  however,  this 
double  cyanide,  like  its  analogues  of  the  other  heavy  metals, 
is  not  a  perfect  complex,  but  in  aqueous  solution  is  slightly 
resolved  into  copper  cyanide  and  potassium  cyanide,  which 
are  further  dissociated  into  their  components.  Hence,  cop- 
per ions  must  be  assumed  as  present  in  the  solution  of  potas- 
sium copper  cyanide;  but  they  are  so  few  in  number  that 
their  presence  cannot  be  chemically  demonstrated.  In  other 
double  cyanides,  e.  g.,  that  of  silver,  the  degree  of  dissocia- 
tion is  sufficient  to  render  possible  a  chemical  test  for  silver 
ions.  There  is  then  a  gradual  transition  from  complex 
salts  to  double  salts.  The  best  means  of  distinguishing  be- 
tween these  two  classes  of  bodies  is  their  electric  behavior. 
This  is  so  because  (the  most  important  consideration)  they 
influence  characteristically  the  pressure  necessary  for  the 
separation  of  the  metal  in  them.  According  to  a  theory 
proposed  by  Nernst  (Z.  f.  ph.  Ch.,  4,  129)  the  potential 
difference  of  a  solid  metal  in  contrast  to  a  liquid  is  dependent 


THEORETICAL    CONSIDERATIONS.  35 

not  only  upon  its  solution-tension,  but  also  upon  the  concen- 
tration of  the  ions  present  in  the  solution ;  it  increases  with 
increasing  dilution.  Just  as  a  solid  in  contrast  with  a  liquid 
shows  a  greater  tendency  to  dissolve,  the  less  of  it  there 
already  is  in  solution  (the  less  in  consequence  is  the  oppos- 
ing osmotic  pressure),  so  a  metal  in  contrast  to  a  liquid 
shows  a  greater  difference  in  potential  the  fewer  ions  there 
are  of  it  in  the  latter.  Conversely,  the  electromotive  force 
intended  to  throw  out  the  metal  ions  in  solution  must,  there- 
fore, be  chosen  larger  in  proportion,  as  it  is  less  supported 
or  aided  by  the  osmotic  pressure  of  the  same,  and  the  less 
also  the  concentration  of  the  ions.  It  must  become  endless 
if  the  number  of  ions  is  infinitely  small.  Therefore,  theo- 
retically speaking,  metals  can  never  be  completely  precipi- 
tated from  their  solutions  by  the  galvanic  current.  Yet, 
as  seen  from  the  formula  of  Nernst,  under  normal  condi- 
tions, the  rise  in  polarization  with  dilution  is  so  very  slow 
that  in  practical  work  it  is  negligible.  In  the  complex  cya- 
nides, however,  the  number  of  metallic  ions  is  so  extremely 
small  that  they  are  capable  of  very  appreciably  influencing 
the  difference  in  potential  requisite  for  their  separation. 
The  degree  of  this  influence  depends,  in  addition  to  the 
specific  property  of  the  double  cyanide,  upon  the  quantity 
of  potassium  cyanide  present  in  the  solution,  inasmuch  as 
the  presence  of  the  latter  retards  the  dissociation  of  the 
metallic  cyanide.  Further,  the  water  may  show  an  abnor- 
mal rise  of  polarization  in  consequence  of  the  small  number 
of  its  ions.  In  neutral  salts,  not  having  ions  similar  to  those 
of  water,  its  decomposition  value  is  about  2.2  volts,  because 
of  the  formation  of  base  and  acid  at  the  electrodes.  Acids 
and  alkalies,  however,  show  normal  pressure.  In  their 
electrolysis,  unlike  that  of  the  alkali  salts,  concentration 
changes  alone  occur  at  the  electrodes.  It  is  therefore  im- 


36  ELECTRO-ANALYSIS. 

portant  with  the  double  cyanides,  in  whose  solutions  the 
higher  decomposition  value  of  water  (2.2  volts)  comes  into 
consideration,  whether  in  them  the  abnormal  potential  of 
the  metals  is  able  to  raise  itself  above  that  of  water,  or 
whether  it  remains  below.  If  the  first  be  the  case,  by  regu- 
lated pressure,  the  hydrogen  alone  will  be  discharged  and 
the  metal  cannot  be  precipitated.  The  number  of  hydrogen 
ions  is,  indeed,  very  small,  but  as  the  number  of  the  metal 
ions  is  also  extremely  small,  therefore,  the  separation  of  the 
former  is  favored  in  consequence  of  their  lower  potential. 

•  "  Precipitation  under  these  conditions  becomes  possible 
only  by  using,  on  the  one  hand,  a  higher  pressure  and  suffi- 
cient current  density,  or,  upon  the  other  hand,  by  decom- 
posing the  potassium  cyanide  present,  thus  lowering  the 
potential  of  the  metal  which  it  is  desired  to  precipitate. 

"  Another  group  of  metals,  namely,  those  sufficiently  dis- 
sociated in  their  double  cyanide  solutions,  are  not  able  to 
raise  their  potential  above  that  of  hydrogen,  hence  they  can 
at  once  be  precipitated  from  a  potassium  cyanide  solution. 

"  The  earlier  view  by  which  the  metals  were  regarded  as 
a  secondary  precipitation,  caused  by  the  potassium  set  free 
by  electrolysis,  leads  to  contradictions.  For  example,  it 
does  not  well  explain  why  the  current  precipitates  some 
metals  readily  from  solutions  containing  an  excess  of  potas- 
sium cyanide,  and  others  only  with  difficulty.  If  it  be  a 
fact  that  potassium  is  discharged  and  it  is  then  in  a  condi- 
tion to  produce  a  secondary  reaction,  why  does  it  act  in  this 
manner  with  certain  metals  and  not  with  the  others  ?  Fur- 
ther, the  intimate  connection,  existing  between  the  precipi- 
tation of  metals  and  their  chemical  detection  by  hydrogen 
sulphide,  argues  most  clearly  in  favor  of  the  first  theory. 

"  This  variation  in  the  behavior  of  metals  in  potassium 
cyanide  solutions  leads  to  another  division,  which  rests  upon 


THEORETICAL    CONSIDERATIONS.  37 

entirely  different  principles,  not  identical  with  those  answer- 
ing for  acid  solutions.  Metals  readily  reduced  from  a 
potassium  cyanide  solution  are  gold,  silver,  mercury  and 
cadmium.  Examples  of  the  opposite  class  are  copper, 
platinum,  arsenic,  nickel,  cobalt,  iron  and  zinc.  It  is  worthy 
of  note  how  the  potential  of  metals,  originally  constant  in 
consequence  of  the  specific  cohesion  of  the  ions,  may  be 
increased  at  will  and  altered  in  its  order  of  magnitude  by 
diminishing  the  number  of  ions. 

'''  There  is  another  instance,  besides  the  double  cyanides, 
which  has  found  practical  application  and  is  explainable  by 
this  same  principle.  Certain  metals,  e.  g.,  arsenic  and  anti- 
mony, able  to  act  both  as  bases  and  acids,  may  be  more  or 
less  completely  robbed  of  their  ionic  condition  by  dissolving 
them  in  alkalies,  thus  imparting  to  them  the  role  of  an  acid. 
Thereby  their  potential  rises  above  that  of  hydrogen  in  a 
manner  perfectly  analogous  to  that  of  the  double  cyanides, 
and  they  are  then  no  longer  reducible  by  the  current. 

"  At  this  point  may  be  recalled  the  fact  which  well  repre- 
sents the  behavior  of  the  metals  upon  electrolysis — it  is  the 
great  analogy  between  their  precipitation  by  the  galvanic 
current  and  by  hydrogen  sulphide.  The  cause  for  this  is 
that  the  tendency  of  metals  and  hydrogen  to  form  ions  in 
general  repeats  itself  in  their  sulphur  derivatives.  In  a  solu- 
tion containing  an  excess  of  hydrogen  ions  there  will  be 
just  as  few  metals  precipitated  by  hydrogen  sulphide  as  by 
the  current  if  the  ionizing  tendency  of  the  metals  is  greater 
than  that  of  .hydrogen.  In  an  alkaline  solution,  in  which 
the  ionizing  tendency  of  the  hydrogen  attains  an  abnormal 
value,  all  those  metals  will  be  precipitated  both  by  the  cur- 
rent and  by  hydrogen  sulphide  whose  ionizing  tendency  is 
lower  than  that  of  hydrogen.  Finally,  in  a  potassium  cya- 
nide solution,  in  which  the  potential  has  been  greatly  in- 


3  ELECTRO-ANALYSIS. 

creased,  only  those  metals  will  be  precipitated  by  hydrogen 
sulphide  which  are  immediately  precipitated  by  the  current. 
True,  the  analogy  between  the  two  series  is  not  absolute  in 
any  sense.  Thus,  hydrogen  sulphide  precipitates  cadmium 
from  a  solution  containing  nitric  acid,  but  this  is  not  the 
case  with  the  current.  But  it  follows  it  in  so  far  that  in 
metallic  mixtures,  hydrogen  sulphide,  as  well  as  the  current, 
causes  a  partial  precipitation.  In  slightly  acid  solutions, 
hydrogen  sulphide  precipitates  cadmium  at  once;  should, 
however,  copper  be  simultaneously  present  in  the  solution, 
at  first  this  metal  only  will  be  precipitated  and  not  until  the 
major  portion  of  it  has  been  thrown  out  of  solution  will 
any  cadmium  appear.  Could,  therefore,  the  action  of 
hydrogen  sulphide  be  regulated  as  the  current  is  regulated, 
a  separation  of  the  two  metals  might  be  possible  in  this 
way. 

"  The  behavior  of  metals  contrasted  with  that  of  hydro-- 
gen in  reference  to  their  potential  in  different  solvents  made 
possible  the  simplest  separations,  and  the  early  methods  were 
almost  exclusively  based  on  this  fact.  Because  the  main- 
tenance of  a  definite  pressure  was  not  necessary,  it  is  nat- 
ural that  it  should  not  occur  that  it  was  important,  hence  it 
was  almost  wholly  ignored.  Formerly,  in  most  precipita- 
tions, equal  voltage  was  used,  and  the  current  strength  was 
regulated  in  accordance  with  the  influence  exerted  by  the 
gas  evolution  upon  the  deposit.  This  was  done  by  the  in- 
troduction or  removal  of  resistances.  Under  particularly 
favorable  conditions,  by  this  means  alone,  metal  separations 
were  effected.  The  current  density  was  so  low  that  the 
ions  of  the  more  readily  reducible  metal  continued  to  the 
end  to  take  upon  themselves  the  discharge  of  electricity,  so 
that  only  after  the  removal  of  the  same  was  it  possible  for 
the  second  metal  to  participate  in  the  electrolysis.  It  is, 


THEORETICAL    CONSIDERATIONS.  39 

however,  in  every  respect  more  practicable  to  lower  the  cur- 
rent density,  not  by  increasing  the  external  resistance  but 
by  lowering  the  pressure,  because  in  this  way  is  not  only  the 
precipitation  of  the  second  metal  prevented,  but  the  current 
density  may  be  allowed  to  increase  appreciably  more  than 
by  the  former  procedure.  Only  arrange  the  pressure  so 
that  it  exceeds  enough  the  polarization  of  the  one  metal 
while  it  continues  below  that  of  the  other.  A  reliable  sepa- 
ration of  metals  may  be  attained  in  this  manner  independ- 
ently of  the  length  of  action  of  the  current. 

"  It  is  obvious  that  the  importance  given  the  pressure,  by 
use  of  this  method,  in  contrast  to  current  density  must  lead 
to  many  alterations  in  regard  to  method  and  apparatus  in 
electrolysis.  First  of  all,  the  oxy-hydrogen  voltameter, 
which  heretofore  has  afforded  us  information  regarding  the 
current  energy  employed,  will  lose  its  importance  as  a  meas- 
uring instrument,  etc." 

Bancroft  ( International  Congress  (1903),  Band  4, 
703),  commenting  upon  the  separation  of  metals  by  atten- 
tion to  their  difference  in  pressure,  adds : 

"  As  a  matter  of  fact,  this  method  is  not  used  in  most 
of  the  standard  separations  which  are  rather  to  be  classed 
as  constant  current  separations,  even  though  the  current  may 
not  be  held  absolutely  constant.  In  order  to  prevent  the 
second  metal  precipitating  as  soon  as  the  first  is  all  down, 
it  is  essential  that  hydrogen  shall  be  set  free  by  the  current 
instead  of  the  second  metal.  The  essential  feature,  there- 
fore, of  a  constant  current  separation  is  that  the  decomposi- 
tion voltage  for  hydrogen  in  any  solution  shall  lie  below 
the  decomposition  voltage  of  one. of  the  two  metals.  Since 
most  separations  were  originally  made  without  a  voltameter 
in  circuit,  no  satisfactory  results  were  obtained  until  a  solu- 
tion was  found  which  permitted  of  a  constant  current  sepa- 


ELECTRO-ANALYSIS. 


ration,  and,  for  this  reason,  all,  except  some  of  the  most 
recent  separations,  are  constant  current  separations." 

Root  (Jr.  phys.  Ch.  (1903),  7,  428),  under  the  direction 
of  Bancroft,  studied  the  conditions  of  a  number  of  metal 
separations  from  solutions  of  cyanides,  oxalates,  phosphates, 
and  tartrates.  The  following  tables  give  most  of  the  im- 
portant separations  for  silver,  mercury,  copper,  bismuth, 
lead,  tin,  nickel,  iron,  cadmium  and  zinc. 


TABLE    I. 


TABLE    II. 


SILVER  OR  MERCURY  FROM 

COPPER  FROM 

Cu 

Nitric  acid 

V 

V 

Bi 

Cyanide  -f-  citrate 

C 

C 

Cyanide 

C 

C 

bismuth  precip- 

Bi 

Nitric  acid 

V 

V 

itates 

Pb 

Excess  nitric  acid 

C 

C 

Pb 

Excess  nitric  acid 

C 

C 

Sn 

Sulphide 

Sn 

NH3-f  tartrate 

C 

C 

(Ag2S  insoluble) 

Fe 

Acid,    phosphate, 

C 

C 

Fe 

Nitric  acid 

C 

C 

or  oxalate 

Cyanide 

C 

C 

Ni 

Acid,  phosphate 

C 

C 

Ni 

Acid 

C 

C 

Oxalate 

V? 

C 

Cyanide 

C 

C 

Cd 

Acid 

V? 

C 

Cd 

Nitric  acid 

C 

C 

Phosphate 

C 

C 

Cyanide 

V? 

C 

Cyanide 

Zn 

Cyanide 

C 

C 

cadmium  precip- 

itates 

C 

C 

Zn 

Acid,  phosphate 

C 

C 

TABLE    III.                                             TABLE    IV. 

BISMUTH  FROM 

IRON  FROM 

Pb 

None 

Ni 

None 

Sn 

NH3  -j-  tartrate 

C 

C 

Cd 

Alkaline    cyanide 

Fe 

Acid  sulphate 

C 

C 

cadmium     pre- 

Ni 

Acid  sulphate 

C 

C 

cipitates 

C 

C 

Cd 

Acid 

C 

C 

Acid   (NH4)2SO4 

Zn 

Acid 

C 

C 

cadmium     pre- 

cipitates 

C 

C 

Phosphate,      cad- 

mium    precipi- 

tates 

C 

C 

Zn 

Alkaline  cyanide, 

zinc    precipi- 

tates 

C 

C 

RAPID    PRECIPITATION    OF    METALS. 


TABLE   V. 


TABLE    VI. 


NICKEL  FROM 

t  ADMIUM  FROM 

Cd 

Alkaline     cyanide 

Zn 

Sulphate 

C 

C 

cadmium     pre- 

Cyanide 

C 

C 

cipitates 

C 

C 

Phosphate 

C 

C 

Acid  (NH4)2S04, 

Oxalate 

C. 

V? 

cadmium     pre- 

cipitates 

C 

C 

Zn 

NaOH  -f  tartrate, 

zinc  precipitates 

C 

C 

"The  first  column  gives  the  metal  and  the  second  the  solu- 
tion. In  the  third  column  C  means  that  a  constant  current 
separation  is  used  and  V  a  voltage  separation.  In  the 
fourth  column  the  same  letters  refer  to  the  method  of  sepa- 
ration as  predicted  from  measurements  of  decomposition 
voltage. 

"  As  was  to  have  been  expected,  practically  all  the  deter- 
minations are  constant  current  separations,  and  the  few  that 
are  not  are  of  minor  importance." 

A  most  interesting  contribution,  along  this  same  line,  has 
been  made  by  Danneel  (Internationaler  Congress  fur  angw. 
Ch.  (1903),  4  Band,  680-687).  Consult  also  Hollard,  Ch. 
N.,  87,  193;  88,  5;  89,  no,  125;  Centralblatt,  I.  (1903), 
600.  See,  further,  F.  Foerster,  Z.  f.  ang.  Ch.,  19  (1906), 
1842-1849.  Ibid.,  29,  1889. 


6.  THE   RAPID    PRECIPITATION    OF    METALS    IN 
THE    ELECTROLYTIC   WAY. 

While  engaged  in  perfecting  old  and  seeking  new  electro- 
methods,  the  writer,  watching  the  precipitation  of  molyb- 
denum in  its  electrolytic  separation  from  tungsten,  observed 
delicate,    blue-colored,    thread-like    masses    extending,    or 
5 


42  ELECTRO-ANALYSIS. 

reaching  out,  from  the  cathode  toward  the  anode — a  flat 
platinum  spiral — which,  as  they  approached  the  latter,  im- 
mediately vanished.  These  threads  of  a  blue-colored  tung- 
sten oxide,  formed  in  the  vicinity  of  the  cathode  by  reduc- 
tion, were  reoxidized  upon  coming  into  the  field  of  oxidation 
surrounding  the  anode.  Immediately  the  thought  sug- 
gested itself  that  by  agitating  the  electrolyte  the  unwished- 
for  reduction  of  the  tungstic  acid  would  not  take  place. 
Then  arose  the  question  as  to  how  this  might  best  be  done. 
The  passage  of  an  air  current  did  not,  for  numerous  rea- 
sons, recommend  itself,  so  that  the  next  thought  was  to 
rotate  the  anode.  This  was  tried.  All  this  occurred  in 
1901.  The  results  were  disappointing.  But  on  applying 
the  idea  in  the  same  year  to  other  metals,  it  was  soon  found 
that  copper,  silver  and  mercury  were  precipitated  in  excel- 
lent form,  and  further,  that  by  causing  the  anode  to  rotate 
at  a  high  speed,  greater  current  intensity  and  higher  voltage 
might  be  applied  with  an  attending,  more  rapid  precipitation 
of  the  respective  metals.  The  time  period  was  astonishingly 
reduced.  The  results  were  carefully  noted,  but  the  earlier 
question  of  the  separation  of  molybdenum  from  tungsten 
continued  to  persistently  obtrude  itself.  Hoping  to  solve  it, 
further  work  with  copper  and  other  metals  along  the  lines 
just  described  was  interrupted  and  not  resumed,  except  at 
short  intervals  in  1902,  until  early  in  1903,  when  the  writer 
directed  Dr.  Franz  F.  Exner,  then  a  student  in  this  labora- 
tory, to  repeat  the  experiments  upon  the  metals,  rotating  the 
anode  while  applying  currents  of  great  intensity  and  high 
voltage.  The  results  of  these  trials  were  embodied  in  Ex- 
ner's  doctoral  thesis  published  in  June,  1903,  and  in  con- 
densed form  in  the  Journal  of  the  American  Chemical 
Society,  Vol.  25,  896.  They  were  of  such  a  remarkable 
character  that  many  chemists  considered  the  field  of  electro- 


RAPID    PRECIPITATION    OF    METALS.  43 

analysis  to  have  been  truly  revolutionized  by  them.  In  the 
opinion  of  the  writer,  they  represent  at  least  a  new  depart- 
ure in  this  domain.  Metals  which,  until  this  study  was  com- 
pleted, were  determined  electrolytically  under  the  most 
favorable  circumstances  (from  o. i  to  0.2  grams)  in  periods 
from  two  to  four  hours  are  now  estimated  in  quantities  vary 
ing  from  0.25  to  0.5  gram  and  more  in  from  five  to  ten  min- 
utes. But  before  discussing  minutely  these  results  of  Exner 
and  those  obtained  along  similar  lines  by  other  students  of 
the  writer,  it  is  proposed  to  sketch  briefly  the  allied  efforts 
of  other  chemists  along  similar  lines. 

The  fact  that  agitation  of  the  electrolyte  favors  the 
electro-deposition  of  metals  has  long  been  recognized  in  the 
great  technical  field  of  electrolysis.  For  some  mysterious 
reason  it  has  not  impressed  itself  very  strongly  upon  the 
minds  of  analysts,  although  it  is  only  just  and  proper  to 
record  that  v.  Klobukow  (J.  pr.  Ch.,  33  (Neue  Folge),  473, 
1886)  particularly  emphasized  the  importance  of  agitating 
the  electrolyte  during  the  passage  of  the  current.  Indeed, 
he  made  this  matter  his  special  study,  devising  various  forms 
of  agitators  to  achieve  his  ends.  He  deprecated  the  blow- 
ing of  gases  through  the  electrolytes  because  it  was  impos- 
sible to  distribute  them  evenly,  and  the  superficial  appear- 
ance of  the  bubbles,  he  thought,  exerted  a  harmful  effect 
upon  the  metal  depositions  near  the  edge  of  the  electrolyte 
and  perhaps  occasioned  undesirable  oxidations.  In  his 
efforts  to  contrive  mechanical  devices  he  rotated  the  cathode 
and  then  the  anode;  indeed,  he  even  held  the  electrodes  sta- 
tionary while  moving  the  electrolyte  itself.  At  last  he 
declared  himself  partial  to  a  rotating  anode  and  announced 
that  the  results  obtained  in  this  way  by  him  in  electrolysis 
were  most  astonishing.  However,  those  results  were  never 
given -to  the  public;  so  that  students  were  permitted  to  rely 


44 


ELECTRO-ANALYSIS. 


on  their  imaginations  to  picture  the  character  of  the  novelty, 
v.  Klobukow's  chief  thought  was  the  agitation  of  the  elec- 
trolyte. The  use  of  high  currents  with  high  speed  of  rota- 

FIG.  12. 


tion  of  the  electrode  was  not  discussed.  In  his  preferred 
form  of  apparatus  a  platinum  dish  served  as  the  cathode. 
The  anode  was  attached  as  shown  in  Fig.  12.  The  power 
was  derived  from  a  water  motor.  The  anode  performed 


RAPID    PRECIPITATION    OF    METALS.  45 

not  more  than  150  revolutions  per  minute.  The  apparatus 
is  sketched  here  because  historically  it  holds  first  place 
among  the  various  forms  of  apparatus  devised  for  agitation 
in  electro-analysis,  and  too  much  credit  cannot  be  given  to 
v.  Klobukow  for  it.  It  is  essentially  the  form  employed 
by  the  author,  by  Exner  and  others  in  this  laboratory, 
v.  Klobukow  used  a  platinum  disk  as  anode. 

FIG.  13. 


Levoir  (Z.  f.  a.  Ch.,  28,  63),  also,  appreciated  the 
advantages  arising  from  agitation  of  the  electrolyte  during 
the  precipitation  of  metals  by  the  current,  for  it  is  to  him 
that  we  are  indebted  for  the  thought  represented  in  the 
apparatus  pictured  in  Fig.  13.  The  positive  electrode  is 


46  ELECTRO-ANALYSIS. 

the  larger  dish ;  in  it  is  suspended  the  smaller  dish — the 
negative  electrode.  By  this  arrangement  it  is  expected  that 
the  electrolyte  will  be  agitated  by  the  oxygen  bubbles  arising 
from  the  positive  electrode,  v.  Klobukow's  criticism  of 
Levoir's  suggestion  was  that  the  requisite  energetic  libera- 
tion of  oxygen  would  not  always  be  attainable  in  metal  pre- 
cipitations; further,  it  may  not  be  advisable  to  have  the 
deposited  metal  come  in  contact  with  oxygen.  Unnecessary 
oxidations  in  the  electrolyte  might  very  easily  occur,  so  that 
all  things  considered,  it  would  seem  wisest  to  utilize  the 
positive  electrode  as  an  agitator,  rotating  it  slowly  about 
its  axis. 

So  far  as  the  writer's  knowledge  extends,  the  idea  of 
Levoir  has  met  with  nothing  like  general  adoption  in 
electro-analysis. 

The  preceding  paragraphs  contain  no  reference  to  the  use 
of  high  currents  and  high  voltage,  which  was  the  dominant 
idea  with  the  writer  and  his  corps  of  students  when  they 
began  in  1901  to  rotate  the  anode  in  electrolysis.  That  is, 
v.  Klobukow  and  Levoir  were  content  to  agitate  the  electro- 
lyte and  to  stop  there.  The  possibility  of  using  higher  inten- 
sity of  current  and  greater  voltage  escaped  their  thought. 

This  idea  first  appeared  in  print  in  an  article  published 
by  Gooch  and  Meclway  (Amer.  Jr.  of  Science  [4th  Series], 
15,  320),  when  they  said: 

"  So  far  as  we  are  aware,  however,  no  attempts  have  been 
made,  heretofore,  to  apply  the  rotary  cathode  in  analytical 
operations,  in  which  it  is  the  object  to  remove  the  metal 
completely  from  solution.  In  such  processes  the  soluble 
anode  is  not  used,  and  the  comparatively  high  electromotive 
force  necessary  to  overcome  the  resistance  and  to  throw 
down  the  metal  with  rapidity  liberates  hydrogen  from  the 
water  solution  simultaneously  with  the  metal,  and  the  con- 


RAPID    PRECIPITATION    OF    METALS. 


47 


FIG.  14. 


TO  REV.  COUNTER 


48  ELECTRO-ANALYSIS. 

sequence  is  the  production  of  a  deposit  lacking  in  compact- 
ness and  adhesiveness.  This  interference  on  the  part  of  the 
evolved  hydrogen  with  the  regularity  of  deposition  appears 
to  be  the  chief  reason  why  low  intensity  of  current  must  be 
used  in  the  ordinary  electrolytic  processes  of  analysis.  We 
have  made  some  experiments,  therefore,  to  see  whether  it 
is  not  possible  to  so  far  avoid  the  interfering  action  of  hydro- 
gen by  the  use  of  the  revolving  cathode  as  to  secure  with 
high  currents  and  in  a  short  time  deposits  sufficiently  adher- 
ent and  homogeneous  for  analytical  purposes." 

The  cathode  was  a  platinum  crucible  of  20  c.c.  capacity. 
It  rotated  at  a  speed  of  from  600  to  800  revolutions  a  min- 
ute. It  was  driven  by  an  electric  motor  fastened  so  that  its 
shaft  was  vertical  (Fig.  14).  The  crucible  was  attached 
to  the  shaft  by  pressing  it  over  a  rubber  stopper  bored  cen- 
trally and  fitted  tightly  on  the  end  of  the  shaft.  "  To  secure 
electrical  connection  between  crucible  and  shaft  a  narrow 
strip  of  sheet  platinum  is  soldered  to  the  shaft  and  then 
bent  upward  along  the  sides  of  the  stopper,  thus  putting  the 
shaft  in  contact  with  the  inside  of  the  crucible  when  the  last 
is  pressed  over  the  stopper.  The  shaft  is  made  in  two  parts 
as  a  matter  of  convenience  in  removing  the  crucible  and  is 
joined,  with  care. to  make  a  good  contact  between  the  two 
pieces  of  shafting,  by  a  rubber  connector  of  sufficient  thick- 
ness to  prevent  the  crucible  from  wabbling  when  rotated." 
A  platinum  plate  was  the  anode.  It  dipped  in  the  salt  solu- 
tion contained  in  the  beaker.  Copper,  silver  and  zinc  salts 
were  studied  in  this  way.  The  results  were  indeed  most 
satisfactory. 

It  must  be  remembered  that  the  cathode  was  rotated  in 
these  trials,  and  when  their  publication  was  made  Exner's 
experiments  were  well  advanced,  results  having  been  ob- 
tained, not  only  with  copper,  zinc  and  silver,  but  with  vari- 


RAPID    PRECIPITATION    OF    METALS. 


49 


ous  other  metals ;  so  that  the  writer  felt  justified  in  privately 
communicating  to  Prof.  Gooch  the  outcome  of  Exner's 
work.  As  the  latter  used  the  rotating-  anode  with  high 
current  and  high  pressure,  suggested  by  the  writer,  and 
Gooch,  the  rotating  cathode,  there  appeared  no  good  reason 
why  each  should  not  continue  to  pursue,  undisturbed,  his 
own  original  plan,  and  this  has  been  done  with  marked  suc- 
cess in  both  cases. 

It  was  only  natural  to  expect  that  modifications  in  forms 
of  apparatus  would  soon  follow.     One  of  the  best  sugges- 

FIG.  15. 


<£ 5?.__. 


tions  in  this  direction  was  that  of  E.  S.  Sheppard  in  the 
Journal  of  Physical  Chemistry,  7,  568.  It  is  used  in  the 
Cornell  Laboratory  (Fig.  15). 

"  Instead  of  a  platinum  crucible,  I  have  used  the  ordinary 

disk  anode,  shortening  the  stem  to  about  6  cm.,  and  fastened 

it  by  a  screw  connector  directly  to  the  shaft  of  the  armature. 

The  connection  to  the  battery  is  made  through  the  iron 

6 


50  ELECTRO-ANALYSIS. 

frame  of  the  motor.  The  motor  used  is  a  toy  motor,  a  very 
poor  affair  in  its  way,  but  sufficient  for  the  purpose,  and 
cheap  enough  to  permit  each  cathode  having  its  own  motor. 
The  use  of  belts  as  suggested  by  Gooch  is  very  unsatisfac- 
tory, owing  to  the  slipping,  etc.  It  was  found  best  to  ar- 
range a  rheostat  for  each  motor,  since  no  two  motors  run 
on  the  same  current,  and  it  is  also  desirable  to  slacken  the 
speed  when  removing  the  beaker  and  washing  the  cathode. 

"  This  rheostat  consisted  of  one  zero,  two  one-ohm  and 
two  two-ohm  coils  connecting  through  the  switch  (S),  the 
other  motor  connection  being  through  the  wire  leading  to 
M,  and  a  no-volt  circuit  lamp  may  of  course  replace  this 
form  of  rheostat. 

"  The  cathode  connection  was  made  through  four  8-volt 
6-C.  P.  lamps  in  multiple  (L)  for  storage  battery  work,  or 
these  are  replaced  by  the  ordinary  no-lamp  for  dynamo 
circuit.  The  current  was  then  regulated  by  loosening  or 
tightening  the  lamps  in  their  sockets.  No  difficulty  was 
experienced  in  getting  a  good  connection  through  the  motor 
frame  to  the  cathode. 

"  The  beaker  containing  the  electrolyte  was  supported  by 
the  wood  support  (C)  on  the  brass  posts  (D).  The  screw 
for  tightening  the  collar  of  (C)  should  be  of  such  a  size 
as  to  allow  manipulating  this  support  with  one  hand,  leav- 
ing the  other  free  to  manage  the  wash  bottle,  etc. 

"  The  anode  was  a  stiff  platinum  wire  held  in  the  usual 
electrode  holder,  connection  being  made  through  the  brass 
posts  (D) .  The  distance  from  the  motors  to  the  base  board 
is  about  30  cm.,  and  between  the  motors  10  cm. 

"  The  disk  electrode  was  used  because  we  happened  to 
have  that  form  in  stock.  A  more  desirable  form  would  be  a 
disk  of  platinum  gauze,  thus  allowing  a  stronger  current  to 
be  used  and  shortening  the  time  required. 


RAPID    PRECIPITATION    OF    METALS.  51 

"  The  brass  conductor  which  connects  the  cathode  to  the 
shaft  is  protected  from  corrosion  by  a  rubber  tube.  A  fin- 
ger stall  does  very  well." . 

Very  satisfactory  determinations  of  the  copper  content  of 
chalcopyrite  and  the  zinc  content  of  sphalerite  were  carried 
out  by  means  of  this  device. 

FIG.  1 6. 


Still  other  schemes  have  appeared  (Fig.  16).  This  is 
taken  from  Perkin's  Practical  Methods  in  Electro-Chemis- 
try. Here : 

"  The  support  for  the  cathode  consists  of  a  gun-metal 
arm,  the  end  of  which  is  drilled  to  allow  a  spindle  to  pass. 
This  spindle  carries  a  small  chuck  (such  as  is  used  in  fixing 
small  drills)  which  is  used  for  holding  the  rotator.  The 
grooved  pulley,  which  is  fastened  on  to  the  upper  end  of  the 
spindle,  bears  on  the  top  of  the  arm,  which  is  ground 
smooth.  The  whole  arrangement  is  driven  by  means  of  a 
belt  from  a  water  turbine  or  electric  motor.  This  arrange- 


5  2  ELECTRO-ANALYSIS. 

ment  is  found  to  give  very  perfect  contact  and  to  work  with 
very  little  friction.  The  parts  should  be  only  slightly  lubri- 
cated, the  best  lubricant  being  a  mixture  of  graphite  and  oil. 
"  The  cathode,  as  is  seen  from  the  figure,  is  a  small  sand- 
blasted cylinder  of  platinum  gauze,  which  has  a  combined 
surface  of  about  25  cm.  The  anode  is  in  the  form  of  a 
double  circle  of  stout  platinum  wire,  and  has  four  little  baf- 
fles placed  at  intervals  around  it,  to  prevent  the  liquid  from 
rotating  with  the  cathode.  A  double  coil  of  stout  platinum 
wire  serves  equally  well.  Of  course  for  peroxide  deposits 
the  rotating  electrode  would  be  the  anode.  A  cylinder  of 
sheet  platinum  also  gives  very  good  results,  but  in  this  case 
very  little  metal  is  deposited  upon  the  inner  surface.  Lon- 
gitudinal slits,  however,  partially  get  over  this  difficulty,  but 
with  gauze  as  shown  in  the  figure  the  deposition  is  practi- 
cally equal  inside  and  outside." 

R.  Amberg  (Z.  f.  Elektrochem.,  10,  853)  and  Fischer  and 
Boddaert  (ibid.,  945)  write  at  some  length  upon  the  rapid 
precipitation  of  metals,  although  their  results  were  in  the 
main  anticipated  by  previous  investigators  in  this  new  field. 
Consult  Sherwood  and  Alleman,  J.  Am.  Ch.  Soc.,  29, 
1065,  upon  the  use  of  tin  as  a  cathode  for  the  rapid  quan- 
titative electrolytic  deposition  of  zinc,  etc. 

As  minute  details  in  the  use  of  the  rotating  anode  will 
be  given  under  the  various  metals,  it  will  not  be  necessary 
here  to  occupy  further  space  for  their  consideration  save  to 
add  that  Henry  Sand  (Z.  f.  Elektrochem.,  10,  452)  remarks, 
in  explanation  of  this  rapid  precipitation  of  metals,  that 
"  it  is  most  probable  the  high  current  densities  are  possible 
and  dependent  solely  upon  the  rapidity  of  renewal  of  the 
liquid  at  the  electrodes.  It  is  extremely  likely  that  in  metal 
precipitation  the  potential  at  the  cathode  is  independent  of 
the  current  density.  The  great  variations  observed  when 


RAPID    PRECIPITATION    OF    METALS.  53 

applying  different  current  densities  are  almost  wholly  the 
consequence  of  local  concentration  changes.  The  great  role 
which  such  changes,  under  circumstances,  can  play  I  showed 
four  years  ago  in  the  electrolysis  of  copper  sulphate  solu- 
tions containing  sulphuric  acid  (Z.  f.  ph.  Ch.,  35,  641). 
Just  as  long  as  copper  ions,  in  appreciable  concentration, 
were  present  at  the  surface  of  the  touched  electrode,  those 
alone  were  precipitated,  when,  however,  they  had  practically 
disappeared  from  this  touched  surface,  all  the  copper  migrat- 
ing in  that  direction  was,  by  diffusion,  set  free  simultane- 
ously with  the  hydrogen.  In  all  instances,  as  a  consequence 
of  local  exhaustion  of  copper  sulphate,  in  spite  of  the  con- 
vection, heating,  hydrogen  evolution,  etc.,  over  60  per  cent, 
of  the  current  was  consumed  in  liberating  hydrogen.  On 
agitating  the  solution  energetically,  copper  alone  was  pre- 
cipitated. Had  the  purpose  of  these  trials  been  to  deter- 
mine copper,  that  metal  would,  in  the  first  instance,  have 
separated  in  a  pulverulent  form ;  in  the  second,  as  a  coherent 
precipitate. 

'  The  conditions  upon  which  the  local  concentration 
changes  at  the  electrode  are  dependent  are  well  known  and 
were  adequately  emphasized  by  Danneel  (Z.  f.  Elektrochem., 
9,  763).  In  the  mind  of  the  writer  of  those  lines,  how- 
ever, in  the  mere  enumeration  of  those  factors,  we  fail  to 
place  their  functions  in  the  true  light.  Thus,  if  it  be  said 
of  diffusion  that  it  acts  in  opposition  to  concentration  alter- 
ations at  the  electrode,  there  is,  thereby,  not  expressed  the 
idea  that  diffusion  renders  possible  current  conductivity, 
and  is  indissolubly  connected  with  it,  and  that  without  dif- 
fusion the  concentration  of  a  metallic  salt  at  the  electrode 
would  fall  at  once  to  zero.  Such  an  enumeration  also 
expresses  just  as  little  the  fact  that  diffusion  alone  without 


54  ELECTRO-ANALYSIS. 

convection  is  never  able  to  completely  cancel  the  alterations 
in  concentration  at  the  electrode. 

"  The  relative  function,  attaching  to  the  individual  fac- 
tors, may  be  best  represented  by  an  expression  for  the  time 
which  expires  until  the  concentration  at  the  electrode  with- 
out any  convection  or  artificial  disturbance  of  the  liquid 
falls  to  zero,  or  at  least  diminishes  by  a  definite  amount. 

"  This  time  period  follows  immediately  from  equation 
2  in  the  cited  article  : 


Here,  Ac  is  the  value  to  which  the  concentration  of  the  salts 
under  consideration  may  fall  (for  analytical  purposes  this 
is  always  the  concentration  of  the  salt)  ;  K  is  the  diffusion 
coefficient  of  the  salt  ;  y  the  number  -~  -*  ^  ®  •*  }-  ;  i  the  current 
density  and  nc  the  conversion  number  of  the  precipitated 
metal  in  the  larger  sense,  i.  e.,  the  ratio  of  the  equivalent  of 
metal,  directed  by  the  current  to  the  cathode,  to  the  entire 
number  of  equivalents  carried  by  the  current.  In  the  case 
of  a  complex  salt  in  which  the  metal  wanders  from  the 
cathode  in  the  form  of  an  anion,  a  negative  value  must  be 
introduced  —  nc.  In  experimenting  with  a  sample  of  copper 
sulphate  containing  free  sulphuric  acid,  it  was  demonstrated 
that  the  expression  is  sufficiently  accurate  when  a  conduct- 
ing electrolyte  is  present.  It  may  easily  happen  that  with 
a  given  apparatus  and  with  a  given  rotation  velocity,  on 
electrolyzing  different  solutions  with  varying  current  densi- 
ties satisfactory  results  will  always  be  obtained  if  the  mag- 
nitude given  above  does  not  exceed  a  definite  value.  The 
expression,  omitting  the  constant  y,  may  be  viewed  as  char- 
acteristic for  the  behavior  of  a  solution  under  electrolysis. 
It  is  evident  from  it  how  far  conducting  salts  favor  decrease 


USE    OF    MERCURY    CATHODE.  55 

in  concentration  (by  reducing  nc),  and  that  in  this  particu- 
lar complex  formation  can  act  more  unfavorably  (by  the 
negative  value  of  nc).  It  may  be  further  concluded  that, 
ceteris  paribus,  at  higher  concentration  of  the  electrolyte,  a 
proportionately  higher  current  density  is  admissible  than  at 
lower  concentration.  In  fact,  in  the  rapid  galvanoplastic 
methods,  solutions  are  applied  in  as  concentrated  form  as 
possible,  with  little  conducting  electrolyte.  In  rapid  analy- 
sis, by  electrolysis,  it  may,  however,  be  advisable  to  keep 
the  volume  as  small  as  possible  and  at  the  same  time  lower 
the  current  strength  and  have  it  as  nearly  proportional  as 
possible  with  the  diminishing  average  concentration.  If 
the  current  strength  be  held  constant,  in  spite  of  decreasing 
concentration,  then  the  efficiency  of  the  stirrer  should  be 
increased  in  inverse  square  ratio  to  the  latter." 

See  also  R.  Amberg,  Z.  f.  Elektrochem.,  10,  385  and  853 ; 
Classen,  Z.  f.  Elektrochem.,  13,  181. 


7.  USE   OF  A  MERCURY   CATHODE. 

LITERATURE. — J.  Am.  Ch.  S.,  25,  884. 

Most  work  in  electro-analysis  has  been  performed  with 
platinum  cathodes.  These  have  had  a  variety  of  shapes : 
dishes,  cones,  cylinders,  gauzes,  etc.  Wolcott  Gibbs  (1880) 
(p.  29)  first  suggested  the  possibility  of  using  metallic  mer- 
cury as  a  cathode.  He  recommended  weighing  out  in  a 
small  beaker  a  definite  amount  of  metallic  mercury  which 
was,  by  means  of  a  platinum  wire,  connected  with  a  battery 
and  made  the  cathode,  while  in  the  salt  solution,  contained 
in  the  beaker,  was  suspended  a  strip  of  platinum,  serving 
as  the  anode.  The  currents  used  varied  greatly  in  strength. 

Three  years  later   (1883)    the  same  chemist   (Am.  Ch. 


56  ELECTRO-ANALYSIS. 

Jr.,  13,  571)  again  directed  attention  to  "  the  employment 
of  mercury  as  negative  electrode,  the  positive  electrode 
being  a  plate  of  platinum.  ...  It  was  found  possible  to 
separate  iron,  cobalt,  nickel,  zinc,  cadmium,  and  copper  so 
completely  from  solutions  of  the  respective  sulphates  that  no 
trace  of  metal  could  be  detected  in  the  liquid  .  .  .  the 
author  had  in  view  both  the  determination  of  the  metal  by 
the  increase  in  weight  of  the  mercury,  and  in  particular  cases 
of  the  molecule  combined  with  the  metal,  either  by  direct 
titration  or  by  known  gravimetric  methods  (p.  29)."  The 
experiments  were  purely  qualitative,  such  being,  in  the 
author's  opinion,  sufficient  to  establish  the  correctness  of 
the  principle  involved. 

In  1886,  Luckow  (Chemiker-Zeitung,  9,  338,  and  Z. 
a.  Ch.,  25,  113),  cognizant  of  the  difficulties  attending 
the  determination  of  zinc  in  the  electrolytic  way,  described 
a  course  (p.  30)  for  this  purpose  which  consisted  in  weigh- 
ing out  in  a  platinum  dish  a  quantity  of  metallic  mercury 
or  its  oxide,  introducing  the  zinc  salt  solution  and  then 
electrolyzing,  when  the  zinc,  combined  with  the  mercury, 
spread  over  the  inner  surface  of  the  dish  as  a  beautiful, 
adherent  amalgam. 

Nothing  further  was  done  towards  developing  the  pre- 
ceding ideas  until  1891,  when  Vortmann  (Ber.,  24,  2749) 
described,  at  considerable  length,  the  determination  of 
several  metals  in  the  form  of  amalgams.  His  plan  con- 
sisted in  adding  a  weighed  quantity  of  mercuric  chloride 
to  the  solution  of  the  salt  to  be  electrolyzed,  the  metals  being 
then  precipitated  together.  The  results  were  quite  interest- 
ing and  seemed  to  offer  decided  advantages,  but  later  experi- 
ence demonstrated  that,  except  in  a  few  cases,  this  method 
of  analysis,  as  elaborated  by  Vortmann,  was  in  nowise  super- 
ior to  the  usual  procedure  in  determining  metals  electrolyt- 
ically. 


USE    OF    MERCURY    CATHODE.  57 

A  few  months  later,  in  the  same  year  (1891),  Drown  and 
McKenna  (Jr.  An.  Ch.,  5,  627),  striving  to  find  a  method 
suitable  for  the  estimation  of  small  amounts  of  aluminium 
in  the  presence  of  a  preponderance  of  iron  (p.  142),  had  re- 
course to  the  suggestion  of  Wolcott  Gibbs.  They  accord- 
ingly weighed  a  beaker  containing  a  layer  of  mercury  (the 
cathode),  and  introduced  into  the  solution  of  the  metals  a 
platinum  plate  (the  anode).  The  current  was  allowed  to 
act  through  the  night  and  the  iron  was  completely  precipi- 
tated in  the  mercury.  Several  difficulties  were  encountered 
in  pursuing  this  course.  The  platinum  wire  projecting  into 
the  mercury  often  had  iron  precipitated  upon  it,  so  that  it 
became  necessary  to  weigh  the  wire,  enclosed  in  a  glass  tube, 
together  with  the  beaker  containing  the  mercury.  Further, 
much  annoyance  was  experienced  in  the  efforts  to  dry  the 
amalgam  and  obtain  constant  weights. 

The  thought  of  the  writer  had  many  times  dwelt  upon  the 
facts  just  mentioned,  until  at  length  it  was  determined  to 
conduct  a  series  of  experiments  with  mercury  as  cathode  to 
establish  two  points  :  (a)  The  determination  of  the  negative 
radical  in  various  salts,  as  well  as  the  metals  combined  with 
them,  and  (b)  the  possibility  of  effecting  the  separation  of 
certain  metals. 

To  this  end,  practically  the  same  device  as  that  used  by 
Drown  and  McKenna  was  adopted.  Into  the  mercury, 
serving  as  cathode,  there  extended  a  glass  tube  from  the 
lower  end  of  which  projected  a  carbon  pencil,  I  mm.  in 
length.  This  pencil  of  carbon  was  preferable  to  the  plati- 
num wire ;  metals  did  not  adhere  to  it ;  and,  therefore,  it  was 
not  necessary  to  weigh  it  together  with  the  beaker  and  the 
mercury.  The  glass  tube  was  nearly  full  of  mercury,  into 
which  dipped  a  copper  wire  connected  with  the  negative 
binding-post.  Such  was  the  form  of  apparatus  first  used, 


ELECTRO-ANALYSIS. 

FIG.  17. 


and  the  results  obtained  were  quite  satisfactory,  although 
difficulty  was  experienced  in  drying  the  amalgam  (J.  Am. 
Ch.  S.,  25,  885).  It  seemed  at  the  beginning  that  this 


USE    OF    MERCURY    CATHODE.  59 

might  prove  deterimental  to  the  general  adoption  of  the 
method  in  ordinary  analysis.  It  was,  however,  successfully 
overcome,  for  it  was  found  that  the  amalgam  could  be 
washed  with  alcohol  and  ether,  thus  removing  the  final  traces 
of  water,  and  that  not  more  than  fifteen  minutes  would  then 
be  necessary  for  the  drying  of  the  metal.  A  number  of  care- 
fully conducted  tests  established  this  point.  In  the  mean- 
time, William  M.  Howard  of  this  laboratory  devised  the 
following  form  of  apparatus  to  eliminate  the  use  of  the 
anode  of  Drown  and  McKenna,  as  well  as  the  carbon  pencil. 

It  is  an  extremely  simple  contrivance,  consisting  of  a 
small  beaker  (50  c.c.  capacity),  (Fig.  17),  near  the  bottom 
of  which  there  is  introduced,  through  the  side,  a  thin  plati- 
num wire.  Internally  it  dips  into  the  mercury,  while  ex- 
ternally it  touches  a  disk  of  sheet-copper  on  which  the  beaker 
rests  and  which  is  connected  with  the  negative  electrode  of 
a  cell,  thus  making  the  mercury  the  cathode.  By  adopting 
this  device  and  by  washing  the  amalgam  with  alcohol  and 
ether,  the  two  chief  disturbing  factors  were  removed. 

How  this  device  was  applied  will  be  indicated  under  the 
several  metals.  Its  modifications  and  uses  in  the  determi- 
nation of  anions  will  be  sufficiently  outlined  in  connection 
with  this  special  chapter  on  electro-analysis. 

Frary  in  a  very  recent  issue  of  the  Z.  f.  Elektrochem. 
(1907),  No.  23,  308,  presents  a  new  form  of  apparatus 
(Fig.  18)  to  be  used  in  the  rapid  precipitation  of  metals. 
A  motor  is  not  necessary.  No  parts  of  the  apparatus  are 
at  any  time  in  motion.  The  parts,  given  in  the  vertical 
section,  are  the  spool  (S),  wound  about  a  cylinder  (E) 
of  thin  sheet  copper  through  which  passes  the  electrolyzing 
current.  The  cylinder  is  large  enough  to  conveniently 
accommodate  a  beaker  (B)  of  150  c.c.  capacity.  The 
spool  is  surrounded,  for  practical  reasons,  with  a  rather 


6o 


ELECTRO-ANALYSIS. 


thick  cylinder  of  sheet  iron  (D),  and  the  entire  system 
placed  on  a  piece  of  sheet  iron  in  order  to  augment  the 
magnetic  field  in  the  beaker.  C  is  the  gauze  cathode.  A 
is  the  anode  of  platinum  wire.  The  electrolyte  must  not 


extend  beyond  the  upper  end  of  the  cathode.  The  spool 
is  made  from  i  kilogram  of  insulated  copper  wire  of  i.i 
mm.  diameter.  Its  resistance  is  about  2  ohms.  The  cath- 
ode may  be  a  cylinder  of  platinum,  silver,  or  copper  gauze. 
Another  device  (Fig.  19),  for  use  with  the  mercury 
cathode,  consists  of  a  U-shaped  electromagnet,  the  spool 
(S)  of  which  is  wound  about  the  bend  of  the  magnet.  In 
the  upper  limb  (pole)  of  the  magnet  is  an  opening  4  cm. 
in  width,  through  the  middle  of  which  passes  an  iron  rod, 
one  centimeter  in  diameter,  leading  to  the  other  pole,  into 
which  it  is  screwed.  The  electrolyzing  vessel  (E)  is  ring- 
shaped  and  fits  into  the  opening  between  the  ring-shaped 
end  of  the  upper  hole  and  the  iron  rod.  A  is  the  ring- 
shaped  anode  of  platinum  wire.  C  is  the  mercury  cathode, 
forming  contact  with  the  copper  plate  (P)  by  means  of  the 


USE    OF    MERCURY    CATHODE. 


6l 


two  platinum  wires.     B  is  a  shield  of  asbestos,  designed 
to  prevent  contact  between  the  plate  and  the  iron  rod. 

In  the  first  apparatus  (Fig.  18)  there  is  a  vertical  mag- 
netic field  with  radial  current  lines,  while  in  the  second 
(Fig.  19)  there  is  a  radial  field  with  vertical  current  lines. 

FIG.  19. 


The  agitation  or  movement  is  particularly  energetic  in  the 
second  form  of  apparatus,  because  of  the  iron  core  and  the 
very  narrow  air  space. 

Frary,  using  the  first  form  of  apparatus,  precipitated 
0.8500  gram  of  copper  from  100  c.c.  of  a  copper  sulphate 
solution,  acidulated  with  ten  drops  of  concentrated  sul- 
phuric acid,  in  fifteen  minutes.  The  current  equalled  6  to 
7  amperes  and  the  pole  pressure  was  about  6  volts. 


62  ELECTRO-ANALYSIS. 

With  the  second  form  of  apparatus  o.i  gram  of  iron  was 
precipitated  from  ferrous  sulphate  in  ten  minutes,  using  a 
current  of  4  amperes. 

See  also  Ashcroft,  Electrochemical  and  Met.  Industry,  4, 

145- 

The  advantages  claimed  by  Frary  for  these  forms  of 
apparatus  are :  they  are  inexpensive ;  they  may  be  run  with- 
out noise,  and  they  require  little  or  no  attention. 

The  writer  inclines  to  the  opinion  that  all  of  these  points 
are  features  of  the  devices  now  in  use  in  this  laboratorv. 


SPECIAL  PART. 


i.  DETERMINATION  OF  THE  DIFFERENT 
METALS. 

COPPER. 

LITERATURE. — Gibbs,  Z.  f.  a.  Ch.,  3,  334;  Boisbaudran,  B.  s.  Ch. 
Paris,  1867,  468;  Merrick,  Am.  Ch.,  2,  136;  Wright  son,  Z.  f.  a.  Ch.. 
15,  299;  Herpin,  Z.  f.  a.  Ch.,  15,  335;  Moniteur  Scientifique  [3  ser.],  5, 
41;  Ohl,  Z.  f.  a.  Ch.,  18,  523;  Classen,  Ber.,  14,  1622,  1627;  Classen 
and  v.  Reiss,  Z.  f.  a.  Ch.,  24,  246;  25,  113;  Hampe,  Berg-Hiitt.  Z.,  21, 
220;  Riche,  Z.  f.  a.  Ch.,  21,  116;  M  akin  tosh,  Am.  Ch.  Jr.,  3,  354; 
Rudorff,  Ber.,  21,  3050;  Z.  £.  ang.  Ch.,  1892,  p.  5;  Luckow,  Z.  f.  a. 
Ch.,  8,  23;  Warwick,  Z.  f.  anorg.  Ch.,  i,  285  ;  Smith,  Am.  Ch.  Jr.,  12, 
329;  Cro  as  dale,  Jr.  An.  Ch.,  5,  133;  Foote,  Am.  Ch.  Jr.,  6,  3335  G.  H. 
Meeker,  Jr.  An.  Ch.,  6,  267;  Classen,  Ber.,  27,  2060;  Heidenreich, 
Ber.,  29,  1585  ;  Regelsberger ,  Z.  f.  ang.  Ch.,  1891,  473  ;  Oettel,  Ch.  Z., 
1894,  879;  Schweder,  Berg-Hiitt.  Z.,  36  (5),  n»  21;  Fernberger  and 
Smith,  J.  Am.  Ch.  S.,  21,  1001  ;  Wagner,  Z.  f.  Elektrochem.,  2,  613; 
Oettel,  Ch.  Z.  (1894),  47,  879;  Foerster  and  Seidel,  Z.  f.  anorg. 
Ch.,  14,  1 06;  Head,  Trans.  Am.  Inst.  Mining  Engineers,  1898;  Rev  ay, 
Z.  f.  Elektrochem.,  4,  313-329;  Ullmann,  Ch.  Z.,  22,  808;  Ho  Hard, 
C.  r.,  123,  1003  (1896)  ;  Kollock,  J.  Am.  Ch.  S.,  21,  923;  Richards  and 
Bisbee,  J.  Am.  Ch.  S.,  26,  530;  Gooch,  Am.  Jr.  Sc.,  xv,  320;  Ch.  News, 
87,  284;  Foerster  and  Coffetti,  Z.  f.  Elektrochem.,  10,  736; 
Denso,  Z.  f.  Elektrochem.,  9,  463;  Medway,  Am.  Jr.  Sc.  [4th  Series], 
xviii,  1 80;  Heath,  J.  Am.  Ch.  S.,  26,  1120-1125;  Spitzer,  Z.  f. 
Elektrochem.,  n,  391;  Koch,  Z.  f.  a.  Ch.,  41,  105;  Danve,  J.  pharm. 
Chim.,  [6],  16,  371;  Kufferath,  Z.  f.  ang.  Ch.,  17,  1785;  Interna- 
tionaler  Congress  fiir  angew.  Ch.,  [Berlin]  Band  4,  677;  Guess,  Eng. 
Min.  Jr.,  81,  328  (1906);  Exner,  J.  Am.  Ch.  S.,  25,  897;  Fischer  and 
Boddaert,  Z.  f.  Elektrochem.,  10,  947;  Foerster,  Z.  f.  ang.  Ch.,  19, 
1890  (1906);  Smith,  J.  Am.  Ch.  S.,  26,  1614;  Kollock  and  Smith, 
Am.  Phil.  Soc.  Pr.,  44,  143;  Flanigen,  J.  Am.  Ch.  S.,  29,  455; 

63 


64 


ELECTRO-ANALYSIS. 


Langness,  ibid.,  29,  460  ;  K o  1 1  o c k  and  Smith,  Am.  Phil.  Soc.  Pr.,  45, 

257- 

Dissolve  19.6  grams  of  pure  copper  sulphate  in  water, 
and  dilute  to  i  liter.  Place  50  c.c.  of  this  solution  (=  0.25 
gram  of  metallic  copper)  in  a  clean  platinum  dish,  pre- 
viously weighed.  Arrange  the  apparatus  as  in  the  ac- 

FIG.  20. 


companying  sketch  (Fig.  20),  the  voltmeter  being  to  the  left 
of  the  dish  and  the  milliamperemeter  and  the  rheostat  to  the 
right-hand  side  of  the  same;  and  having  done  this,  add  9-10 
drops  of  concentrated  nitric  acid  to  the  solution  of  the 
electrolyte;  dilute  to  125  c.c.  with  water;  heat  to  70°,  and 
electrolyze  with  a  current  of  N.D100  =  0.09  ampere  and  1.9 
volts.  Cover  the  vessel  with  a  perforated  watch-crystal 
during  the  decomposition.  Four  to  five  hours  will  suffice  for 
the  precipitation.  To  ascertain  when  the  metal  has  been 
completely  precipitated,  add  water  to  the  dish;  this  will 
expose  a  clean,  platinum  surface,  and  if  in  the  course  of  half 


DETERMINATION    OF    METALS COPPER.  65 

an  hour  no  copper  appears  upon  it,  the  deposition  may  be 
considered  as  finished.  Or,  a  drop  of  the  liquid  may  be 
removed  and  brought  in  contact  with  a  drop  of  ammonium 
hydroxide  or  hydrogen  sulphide,  when,  if  a  blue  coloration 
or  black  precipitate  is  not  produced,  the  deposition  can  be 
considered  ended. 

As  the  precipitation  has  been  made  in  an  acid  solution  the 
current  should  not  be  interrupted  until  the  acid  liquid  has 
been  removed,  for  in  many  cases  the  brief  period  during 
which  the  acid  can  act  upon  the  metal  will  be  sufficient  to 
cause  some  of  the  latter  to  pass  into  solution.  To  obviate 
this,  siphon  off  the  acid  liquid.  As  the  acidulated  water  is 
conveyed  away  by  the  siphon,  pour  distilled  water  into  the 
dish.  Empty  the  platinum  dish  twice  in  this  way ;  the  cur- 
rent can  then  be  interrupted  without  loss  of  copper. 
Finally,  disconnect  the  dish,  wash  the  deposit  with  hot 
water  and  then  with  alcohol.  Dry  the  precipitated  copper  at 
a  temperature  not  exceeding  100°  C. ;  an  air-bath,  an  asbes- 
tos plate,  or  warm  iron  plate  will  answer  for  this  purpose. 
Do  not  weigh  the  dish  until  it  is  perfectly  cold,  and  has  at- 
tained the  temperature  of  the  balance-room. 

In  heating  the  dish  containing  the  electrolyte,  do  not  apply 
a  direct  lamp  flame;  attach  a  circular  piece  of  thin  sheet- 
asbestos  to  the  lower  side  of  the  ring,  supporting  the  plati- 
num dish,  and  under  it  place  an  ordinary  Bunsen  burner,  or 
one  reduced  in  size.  Water-baths  are  not  needed  for  heat- 
ing purposes. 

Riidorff  suggests  the  addition  of  ten  drops  of  a  saturated 
sodium  acetate  solution  to  the  acid  liquid  from  which  the 
copper  has  been  precipitated  before  interrupting  the  current. 
The  acetic  acid,  which  is  liberated,  will  not  immediately  at- 
tack the  copper,  which  can  be  at  once  washed  and  treated  as 
just  described. 
7 


66  ELECTRO-ANALYSIS. 

Copper  is  very  readily  precipitated  from  solutions  con- 
taining free  nitric  or  sulphuric  acid.  Hydrochloric  acid 
should  never  be  used. 

A  platinum  dish,  50  mm.  in  diameter  and  20  mm.  in  depth, 
may  be  substituted  for  the  spiral  anode.  There  are  openings 
in  the  dish  to  facilitate  circulation  and  accelerate  the  precipi- 
tation of  the  metal. 

The  deposition  of  the  copper  can  also  be  made  in  a  plati- 
num crucible,  or  upon  the  exterior  surface  of  the  same. 
This  is  sometimes  convenient.  Place  the  liquid  undergoing 
electrolysis  in  a  beaker  (capacity  100-250  c.c.),  and  suspend 
the  crucible  in  it,  supporting  it  there  by  a  tight-fitting  cork, 
through  which  passes  a  stout  copper  wire,  in  connection  with 
the  negative  electrode  of  a  battery.  The  positive  electrode 
is  a  platinum  plate  projecting  into  the  liquid.  The  end  of 
the  decomposition  may  be  learned  by  adding  water  to  the 
solution  in  the  beaker.  No  further  appearance  of  copper  on 
the  newly  exposed  platinum  indicates  the  end  of  the  precipi- 
tation. Raise  the  crucible  from  the  liquid,  wash  the  copper 
with  water,  then  detach  the  vessel  carefully  from  the  cork, 
and  dry  as  already  directed. 

If  the  current  be  permitted  to  act  too  long  in  the  presence 
of  sulphuric  acid,  copper  sulphide  may  be  produced.  Black 
spots  on  the  surface  of  the  copper  deposit  indicate  this. 

Instead  of  using  either  of  the  suggestions  first  offered, 
substitute  the  apparatus  of  Riche  if  convenient.  This  con- 
sists in  suspending  a  crucible  within  a  crucible.  The  sides 
of  the  inner  vessel  are  perforated  so  that  the  liquid  will 
maintain  uniform  concentration.  It  is  practically  the  same 
as  the  device  just  described  above. 

Engels  recommends  the  addition  of  urea  or  hydroxyl- 
amine  sulphate  to  the  copper  sulphate  solution,  as  it  seems 
to  favor  the  deposition  of  the  metal.  He,  therefore,  pro- 


DETERMINATION    OF    METALS COPPER.  6/ 

ceeds  as  follows:  Add  10—15  c<c-  °f  concentrated  sulphuric 
acid  and  1.5  grams  of  hydroxylamine  sulphate,  or  i  gram 
of  urea,  to  the  salt  solution,  dilute  to  150  c.c.  with  water, 
heat  to  70°,  and  electrolyze  with  a  current  of  N.D100  =  0.8- 
i.o  ampere  and  2.7-3.1  volts.  The  metal  will  be  precipi- 
tated in  one  and  one-half  hours. 

Copper  can  also  be  precipitated  from  the  solution  of 
ammonium-copper  oxalate.  To  this  end  the  copper  solution 
(sulphate  or  chloride)  is  treated  with  an  excess  of  a  satu- 
rated solution  of  ammonium  oxalate  diluted  to  120  c.c.  with 
water;  heated  to  60°  and  electrolyzed  with  N.D100  =  0.35-- 
i.o  ampere  and  2.5  to  3.2  volts.  As  the  metal  begins  to  sepa- 
rate, and  the  original  deep  blue  color  of  the  liquid  disappears, 
add  20-30  c.c.  of  a  cold  saturated  solution  of  oxalic  acid. 
This  should  be  added  gradually  from  a  burette.  Avoid  the 
precipitation  of  insoluble  copper  oxalate.  When  the  decom- 
position is  finished,  decant  the  solution,  and  wash  the  deposit 
of  copper  repeatedly  with  water  and  then  with  alcohol.  Dry 
as  previously  directed.  The  precipitation  is  generally  com- 
plete after  three  hours.  Use  ferrocyanide  of  potassium  to 
learn  whether  all  the  metal  has  been  precipitated. 

E.  Wagner  recommends  the  following  procedure  in  the 
precipitation  of  copper  from  an  oxalate  solution :  Pour  the 
copper  solution  into  the  ammonium  oxalate  solution  (4 
grams  of  ammonium  oxalate  in  60  grams  of  water  for 
I  gram  of  copper  sulphate)  ;  at  the  beginning  electrolyze 
with  a  current  of  0.05  ampere  for  one-half  hour,  then  in- 
troduce 5  c.c.  of  a  cold  saturated  solution  of  oxalic  acid, 
and  at  the  expiration  of  five  minutes  increase  the  current 
to  0.3  ampere.  The  temperature  of  the  electrolyte  should 
equal  60°.  In  the  following  eighty  minutes,  during  four 
intervals,  5  c.c.  of  oxalic  acid  are  added  at  each  period  and 
the  maximum  current  of  0.4  ampere  is  applied.  Two  hours 


68 


ELECTRO-ANALYSIS. 


after  the  close  of  the  circuit  neither  ammonia  nor  potassium 
ferrocyanide  will  show  the  copper  reaction  with  the  solution. 
The  liquid  should  be  siphoned  off  without  the  interruption 
of  the  current.  The  deposit  of  copper  should  be  washed  and 
dried  as  previously  indicated. 

Copper  can  also  be  determined  quite  accurately  in  solu- 
tions of  the  phosphate  in  the  presence  of  free  phosphoric 
acid,  or  in  a  formate  solution  containing  free  formic  acid. 

The  following  example  is  given  to  show  the  applicability 
of  an  acid  phosphate  solution  for  this  particular  purpose  • 
To  a  solution  of  copper  sulphate  (  =0.1239  gram  of  cop- 
per) were  added  20  c.c.  of  a  solution  of  disodium  hydrogen 
phosphate  (sp.  gr.  1.0358)  and  5  c.c.  of  phosphoric  acid 
(sp.  gr.  1.347).  It  was  then  diluted  to  225  c.c.  with  water, 
heated  to  65°,  and  electrolyzed  with  a  current  of  N.D100  = 
0.035-0.068  ampere  and  2.2-2.6  volts.  The  precipitation 
was  completed  in  six  hours.  The  deposit  of  copper  weighed 
0.1238  gram.  It  was  washed  and  dried  as  previously  di- 
rected, p.  65. 

Riidorff  obtained  excellent  results  with  the  following  con- 
ditions :  0.1-0.3  gram  of  metallic  copper  in  150  c.c.  of  water, 
to  which  were  added  2-3  grams  of  potassium  or  ammonium 
nitrate  and  20  c.c.  of  ammonium  hydroxide  (0.91  sp.  gr.). 
Electrolyze  at  the  ordinary  temperature  with  a  current  of 
N.D100  =  i  ampere  and  3.3-3.6  volts.  It  is  claimed  that 
by  observing  the  preceding  conditions  copper  can  be  fully 
precipitated  in  the  presence  of  chlorides.  An  excess  of  ace- 
tic acid  should  be  added  to  the  solution  before  the  current  is 
interrupted. 

Oettel  remarks  on  the  precipitation  of  copper  from 
ammoniacal  solutions  that  the  metal  can  be  quantitatively 
deposited  from  a  slightly  ammoniacal  liquid,  containing 
ammonium  nitrate,  with  a  current  density  of  0.07-0.27 


DETERMINATION    OF    METALS COPPER. 


69 


ampere  per  square  decimeter.  When  ammonium  nitrate  is 
absent  and  the  quantity  of  ammonia  is  large,  the  metal  de- 
posits become  spongy.  He  found  the  most  satisfactory 
concentration  to  be  0.8  gram  of  copper  for  100  c.c.  of  liquid 
when  using  a  wire-form  anode  with  a  cylinder  or  cone  as 
cathode.  Chlorine,  zinc,  arsenic,  and  small  amounts  of 

FIG.  21. 


antimony  were  without  deleterious  effect.  In  the  presence 
of  lead,  bismuth,  mercury,  cadmium  and  nickel  the  results 
were  high. 

Moore  advises  dissolving  the  recently  precipitated  copper 
sulphide,  obtained  in  the  ordinary  course  of  analysis,  in 
potassium  cyanide;  and,  after  the  addition  of  an  excess  of 
ammonium  carbonate,  electrolyzes  the  warm  (70°)  solution. 
In  using  this  electrolyte  care  should  be  taken  to  interrupt  the 


;o 


ELECTRO-ANALYSIS. 


current  just  as  soon  as  the  copper  has  been  fully  precipitated, 
otherwise  metallic  platinum  may  be  deposited  upon  the 
copper. 

In  this  laboratory  it  was  observed  that  the  electrolysis 
can  be  best  and  most  satisfactorily  executed  by  dissolving 
the  sulphide  in  as  small  a  volume  of  potassium  cyanide  as 
possible,  diluting  to  150  c.c.  with  water,  heating  to  65°, 

FIG.  22. 


and  electrolyzing  with  N.D100  =  0.15-0.8  ampere  and 
3-4.5  volts.  The  metal  will  be  fully  precipitated  in  from 
two  to  three  hours. 

It  has  been  asserted  from  time  to  time  that  in  an  alkaline 
cyanide  solution  there  is  great  probability  that  the  anode  will 
suffer  loss  and  that  the  dissolved  platinum  will  reappear  in 
the  cathode.  This  point  has  been  most  carefully  considered 


DETERMINATION  OF  METALS — COPPER. 


in  this  laboratory  with  the  result  that  if  the  quantity  of 
cyanide  added  to  the  copper  solution  be  not  more  than 
enough  to  precipitate  and  redissolve  the  metallic  cyanide 
there  will  be  no  solution  of  the  platinum  anode.  Heating 
the  solution  to  65°  favors  the  deposition  of  the  copper.  It 
was  further  ascertained  that  in  the  presence  of  a  definite 
amount  of  ammonium  hydroxide  there  is  absolutely  no  loss 
sustained  by  the  anode  in  the  cyanide  electrolyte,  and  that 
the  precipitation  of  metal  is  much  accelerated.  Two  ex- 
amples illustrate  this : 


COPPER 

IN 

GRAMS. 

POTASSIUM 
CYANIDE 
IN  GRAMS. 

AMMONIUM 
HYDROXIDE 

IN   C.C. 

N.  DIOO 
AMP. 

VOLTS. 

TEMPERA- 
TURE. 

TIME 

IN 

HOURS. 

GRAMS  OF 
COPPER 
FOUND. 

0.2015 

i-5 

IO 

I.OO 

5 

65 

I 

0.2014 

0.2015 

i*5 

IO 

0.66 

5 

65 

I 

0.2015 

FIG.  23. 


72  ELECTRO-ANALYSIS. 

In  the  analysis  of  commercial  copper  Luckow  employed 
the  apparatus  pictured  in  Fig.  21.  The  beaker1  contains  the 
electrolyte,  and  the  metal  is  precipitated  upon  the  cylinder 
of  platinum.  It  is  a  very  satisfactory  device  for  almost  any 
kind  of  electrolytic  work.  Either  one  of  the  arrangements 
pictured  in  Figs.  22  and  23  will  answer  for  the  same  pur- 
pose. The  platinum  gauze  cathode  in  Fig.  23  is  much 
favored  by  analysts.  An  anode  of  similar  material  and  form 
can  be  used  to  advantage.  To  calculate  the  approximate 
surface  of  a  cylindrical  gauze  cathode  use  the  formula 

5=  nd2v'nlb 

in  which  d  is  the  diameter  of  the  wire,  n  the  number  of 
meshes  per  square  centimeter,  /  the  length  and  b  the  width 
of  the  strip  of  gauze  used  (height  of  the  cylinder). 
(Winkler,  Ber.,  32,  2192.) 

The  Rapid  Precipitation  of  Copper  With  the  Use  of 
a  Rotating  Anode. 

Arrange  the  apparatus  and  dish  as  pictured  on  p.  44. 
Use  an  anode  of  the  form  in  Fig.  24.  To  the  solution  of 
the  copper  salt,  placed  in  the  dish,  add  one  cubic  centimeter 
of  dilute  sulphuric  acid  (i  :  10),  dilute  the  solution  to  125 
c.c.,  thus  exposing  a  cathode  area  of  100  sq.  cm.,  cover  the 
dish  with  suitable  glass  covers,  heat  the  liquid  almost  to 
boiling,  remove  the  lamp,  start  the  rotator,  giving  the  anode 
a  speed  of  600  to  700  revolutions  per  minute,  and  let  a  cur- 
rent of  five  amperes  and  five  volts  pass.  When  the  electro- 
lysis is  complete  (indicated  by  the  colorless  solution),  stop 
the  rotator,  and  reduce  the  current  by  throwing  in  resistance 
from  the  rheostat.  Add  distilled  water  to  cover  any  ex- 
posed metal  and  thus  prevent  oxidation.  Siphon  off  the 
acid  liquor,  keeping  the  dish,  however,  full  by  the  addition 


DETERMINATION    OF    METALS COPPER. 


73 


of  water  from  a  wash  bottle.  Disconnect  the  dish,  wash 
the  deposit  of  copper  with  warm  water,  alcohol  and  ether. 
Dry  and  weigh.  With  the  conditions  just  outlined,  0.4994 
gram  of  metal  was  frequently  deposited  in  five  minutes. 
Miss  Langness,  working  in  this  laboratory,  precipitated 


FIG.  24. 


FIG.  25. 


0.5035  gram  of  copper  in  seven  minutes  by  the  use  of  ten 
volts  and  5  to  13  amperes.     The  deposits  of  metal  were 
perfectly  adherent,  dark  red  in  color  and  had  a  beautiful 
velvet-like  appearance. 
Rate  of  precipitation: 

In   i    minute 0.1493  gram  of  metal 

In  2   minutes 0.3019  gram  of  metal 

In  3   minutes 0.4371   gram  of  metal 

In  4   minutes 0.4925  gram  of  metal 

In  5   minutes 0.5029  gram  of  metal 

Or,  there  may  be  used  a  dish   (Langness,  J.  Am.  Ch. 

S.,  29,  460)    anode  with  the  form  shown  in  Fig.   25   so 

constructed  as  to  be  about  7  cm.  in  diameter  and  3  cm.  deep. 

conforming  throughout  with  the  cathode.     In  its  sides  are 

8 


74 


ELECTRO-ANALYSIS. 


ten  slits  perpendicular  to  the  edge,  each  slit  being  1.8  cm. 
long  and  0.5  cm.  wide.  Free  circulation  of  the  electrolyte 
is  insured  by  these  openings  and  through  a  circular  open- 
ing, 1.3  cm.  in  diameter,  in  the  bottom  of  the  dish.  The 
anode  is  held  in  position  by  a  stout  platinum  rod.  The 
anode  is  so  adjusted  that  it  is  equidistant  from  the  sides  of 
the  cathode.  The  electrolyte,  during  the  rotation  of  the 
anode,  is  all  contained  within  the  space  bounded  by  the 
cathode  and  the  outer  surface  of  the  anode.  There  is  none 
within  the  inner  dish.  The  dilution,  therefore,  is  less  than 
when  using  a  spiral  anode.  When  properly  adjusted  this 
anode  occasions  absolutely  no  splashing  and  no  loss  of 
electrolyte  is  sustained.  To  show  the  result,  on  employing 
this  anode,  five  actual  experiments  are  here  introduced : 


No. 

("u  PRESENT 
IN  GRAMS. 

VOLTS. 

AMPERES. 

TIME,  MIN. 

WT.  OF  (  u  IN 
GRAMS 

I 

0.4884 

7+ 

IO-I5 

4 

0.4883 

2 

0.4884 

8 

10-15 

3 

0.4884 

3 

0.4884       |          8 

10-15 

5 

0.4887 

4 

0.4884 

8 

10 

2 

0.4634 

5 

0.4884 

8 

10 

I 

O.2OIO 

The  electrolyte  in  each  instance  did  not  exceed  sixty  cubic 
centimeters  in  volume.  The  character  of  the  metal  deposits 
was  the  same  as  when  using  the  spiral  anode.  The  volume 
of  free  sulphuric  (i:  10)  was  i  c.c.  in  all  the  trials  just 
described. 

It  may  be  preferred  to  use  a  nitric  acid  electrolyte.  If  so, 
proper  working  conditions  can  be  readily  formed  by  obser- 
vation of  the  following  experiments: 


DETERMINATION    OF    METALS COPPER. 


75 


No. 

COPPER 
PRESENT 
IN  GRAMS. 

ACID  IN 
c.c    SP.  GR. 

I  2. 

DILUTION 

IN  C  C. 

VOLTS. 

AMPERES. 

TIME  IN 

MINUTES. 

COPPER 
IN  GRAMS 
FOUND. 

I 

0.4876 

0-5 

125 

8 

7 

15 

0.4878 

2 

0.4876 

0-5 

I25 

8 

7 

15 

0.4877 

3 

0.4876 

o-5 

125 

8 

8 

15 

0.4875 

4 

0.4876 

0-5 

125 

8 

8 

IO 

0.4875 

The  spiral  anode  was  used  in  these  trials.     The  metal  de- 
posits were  brilliant,  adherent  and  crystalline. 
Rate  of  precipitation: 

In    i  minute 0.1507  gram  of  metal 

In    2  minutes 0.25 1 8  gram  of  metal 

In    3  minutes 0.3418  gram  of  metal 

In    4  minutes 0.3960  gram  of  metal 

In    5  minutes 0.4486  gram  of  metal 

In    6  minutes 0.4654  gram  of  metal 

In    8  minutes 0.4852  gram  of  metal 

In  10  minutes 0.4875  gram  of  metal 

See  also  J.  Am.  Chem.  S.,  25,  898. 

In  an  ammoniacal  electrolyte,  containing  0.4967  gram  of 
copper,  1.2  gram  of  ammonium  nitrate,  total  dilution  125 
c.c.,  a  current  of  9  amperes  and  8  volts,  using  the  rotating 
spiral  anode,  precipitated  0.4963  grams  of  metal  in  fifteen 
minutes.  The  deposits  were  perfectly  adherent  and  very 
bright  in  color.  In  this  same  electrolyte,  if  the  dish  anode 
be  substituted  and  a  current  of  seventeen  amperes  and  six 
volts  be  employed,  0.4824  gram  of  copper  can  be  com- 
fortably precipitated  in  six  minutes.  (See  also  J.  Am. 
Chem.  S.,  25,  898.) 

The  preceding  conditions  answer  well  for  the  determi- 
nation of  copper  in  chalcopyrite.  The  latter  having  been 
reduced  to  a  fine  powder  is  rapidly  decomposed  in  a  small 
beaker  by  boiling  with  concentrated  nitric  acid.  When  the 


76  ELECTRO-ANALYSIS. 

decomposition  is  complete  the  solution  is  quickly  evaporated 
to  dryness,  the  residue  moistened  by  a  few  drops  of  pure 
nitric  acid,  water  added,  the  solution  heated  and  then  fil- 
tered into  a  weighed  platinum  dish  where  it  is  mixed  with 
an  excess  of  ammonium  hydroxide.  The  iron  will,  of 
course,  be  precipitated  as  hydroxide  but  without  paying- 
attention  to  it  the  anode  is  put  in  motion  and  the  solution 
electrolyzed.  There  is  no  danger  of  any  of  the  ferric 
hydroxide  attaching  to  the  deposit  of  copper.  The  thorough 
agitation  of  the  electrolyte  prevents  this.  Numerous  de- 
terminations have  been  made  in  this  laboratory  and  the  re- 
sults have  been  most  concordant.  Of  course  if  the  plan  is 
not  approved  by  the  analyst  ammonium  hydroxide  may  be 
added  directly  to  the  acidulated  (HNO3)  water  solution 
of  the  mineral  before  filtering  out  the  gangue,  thus  bring- 
ing the  latter  and  the  resulting  ferric  hydroxide  together 
upon  the  filter.  The  blue  colored  ammoniacal  filtrate  will 
contain  an  abundance  of  ammonium  nitrate  so  that  one  may 
proceed  at  once  with  its  electrolysis  as  just  directed. 

An  advantage  possessed  by  this  electrolyte  is  that  in  the 
ordinary  course  of  analysis  copper  is  very  frequently  got 
in  the  form  of  nitrate.  See  separation  of  copper  from 
nickel  (p.  197). 

From  an  alkaline  cyanide  electrolyte  the  precipitation  of 
copper  proceeds  rapidly  and  well.  Thus,  to  a  solution  con- 
taining 0.2484  gram  of  metal  there  was  added  just  enough 
potassium  cyanide  to  precipitate  copper  cyanide  and  then 
dissolve  it.  On  dilution,  the  liquid,  being  brought  to  boil- 
ing, was  electrolyzed  with  a  current  of  N.D100  =  6  amperes 
and  1 8  volts.  The  precipitation  was  complete  in  eighteen 
minutes.  The  deposit  was  deep  red  in  color  and  shone  as 
if  it  had  been  polished.  The  deposition  of  metal  from  this 
electrolyte  is  even  more  rapid,  when  using  the  dish  anode 


DETERMINATION    OF    METALS COPPER.  77 

(p.  73).  Thus,  to  a  solution  of  potassium  copper  cyanide 
(  :=  0.4882  gram  of  copper)  were  added  10  c.c.  of  ammo- 
nium hydroxide  (sp.  gr.  0.93  at  24°)  and  it  was  electrolyzed 
with  a  current  of  15  amperes  and  seven  volts.  In  a  period 
of  six  minutes  0.4883  gram  of  copper  was  precipitated. 

Here,  again  is  an  admirable  means  of  determining  the 
copper  content  of  minerals.  Boil  down  to  dryness  a 
weighed  (0.5  gram)  amount,  for  example,  of  finely  divided 
chalcopyrite  with  aqua  regia.  Take  up  the  residue  with  a 
little  hydrochloric  acid  and  water;  filter  and  supersaturate 
the  filtrate  writh  hydrogen  sulphide  gas ;  filter  out  the  copper 
sulphide  and  having  washed  it  with  hydrogen  sulphide 
water,  dissolve  it  from  off  the  filter  in  as  little  warm  dilute 
potassium  cyanide  as  possible,  collect  the  cyanide  filtrate  in 
a  weighed  platinum  dish  and  electrolyze  as  directed  in  the 
preceding  paragraph.  The  results  will  be  perfectly  satis- 
factory. 

The  Rapid  Precipitation  of  Copper  With  the  Use  of 
the  Rotating  Anode  and  Mercury  Cathode  (J.  Am.  Ch. 
S.,  25,  883;  J.  Am.  Ch.  S.,  26,  1595;  ibid.,  26,  1614;  Am. 
Phil.  Soc.  Pr.,  XLIV.  (1905),  137;  J.  Am.  Ch.  S.,  27, 
1527;  Myers,  J.  Am.  Ch.  S.,  26,  1124). 

In  the  introduction  (p.  58)  reference  was  made  to  the 
form  of  cell  or  cup  which  may  be  used  with  advantage  when 
mercury  is  applied  as  a  cathode  in  electro-analysis.  Such 
cups  can  easily  be  made  from  ten-inch  test  tubes  of  soft 
glass.  Into  a  tube  of  this  kind  introduce  a  layer  of  mercury 
sufficient  to  cover  the  platinum  wire  fused  through  the  bot- 
tom or  side  of  the  cup.  Re-weigh  the  cup,  place  it  upon  a 
plate  of  sheet  copper,  connected  with  the  negative  electrode 
of  a  battery,  whereby  the  mercury  becomes  the  cathode. 
Introduce  a  solution  of  copper  sulphate,  add  a  drop  or  two 
of  sulphuric  acid  and  suspend  the  anode  (see  p.  58)  from 


/  ELECTRO-ANALYSIS. 

the  rotator.  Provide  the  cup  with  cover-glasses,  notched 
so  as  to  allow  the  passage  of  the  anode.  These  glasses  can 
be  readily  made  from  the  slides  used  in  microscopic  work. 

The  anode  is  now  rotated  precisely  as  when  making  pre- 
cipitations upon  a  platinum  dish  cathode  (p.  72).  When 
high  currents  are  used  the  solution  of  the  metal  will  fre- 
quently be  heated  to  boiling.  Some  of  the  liquid  will,  of 
course,  be  carried  to  the  sides  of  the  cup  and  to  the  cover 
glasses  by  the  escaping  gases  or  by  the  agitation  of  the 
liquid.  Experience  has  shown  that  it  is  not  necessary  to 
wash  down  this  portion,  because  the  condensed  steam  con- 
tinually frees  the  sides  from  the  solution.  The  cover- 
glasses  should  now  and  then  be  tilted  against  the  sides  of 
the  tube  in  order  to  run  off  the  water  which  collects  in  large 
drops. 

It  has  been  repeatedly  observed  that  the  greater  the  con- 
centration of  the  electrolyte,  the  greater  the  rapidity  of  depo- 
sition, but  the  last  traces  of  metal  separate  slowly,  so  after 
a  solution  has  become  colorless,  continue  the  electrolytic 
action  several  minutes  in  order  to  precipitate  the  minute 
amount  remaining  unprecipitated. 

When  the  metal  has  been  completely  deposited,  stop  the 
rotator,  remove  the  cover-glasses  and  fill  the  decomposition 
cell  with  distilled  water.  This  should  then  be  siphoned  off 
to  the  level  of  the  spiral  and  the  liquid  replaced  by  distilled 
water  until  the  current  drops  to  zero.  This  wash  water 
should  always  be  put  aside  and  tested  to  ascertain  that  the 
metal  has  been  completely  removed.  Next  interrupt  the 
current,  remove  the  tube  and  wash  its  contents  again  with 
distilled  water,  inclining  and  twirling  the  cell  in  order  to 
more  completely  wash  the  amalgam.  As  much  of  the  water 
as  possible  should  be  poured  from  the  cell  and  the  amalgam 
then  be  washed  twice  with  absolute  alcohol  and  twice  with 


DETERMINATION    OF    METALS COPPER. 


79 


ether.  It  should  be  wiped  dry  on  the  outside  and  after  the 
volatilization  of  the  ether  be  placed  in  the  desiccator  and 
weighed  as  previously  described. 

The  following  experiments  are  taken  from  a  laboratory 
notebook.  They  show  that  by  the  method  just  described 
rapidity  and  accuracy  are  obtained  without  any  difficulty 
whatsoever.  Even  inexperienced  chemists  get  very  satis- 
factory estimations  not  only  of  copper,  but  of  other  metals, 
as  will  be  observed  later. 


h 

0 

% 

U  2 

S5         W 

2 

«  "  </i 

«    ID 

H 

*i 

2  g  S 

2  S3 

§S 

2  • 

6 

£  z  ^ 

B  «  t3 

3u 

£  u 

H 

s  z  5 

W  D 

§tJ 

§^ 

•3p 

Pi 

Ir 

O 

PH       PH 

II 

£° 

r°    " 

i£ 

w 

i 

0.7890 

.25 

12 

3-5 

6 

1200 

10 

0.7900 

-LO.OOI 

2 

0.3945 

12 

4 

6 

1081 

5 

0.3941 

—0.0004 

3 

0.3945 

'25 

12 

3-5 

6 

1200 

6 

0-3942 

—0.0003 

4 

0-3945 

12 

5 

6.5 

1200 

5 

0.3944 

-O.OOOI 

5 

0-3945 

.00 

10 

2-4 

9-7 

1200 

6 

0.3946 

+  0.0001 

6 

0.3945 

.17 

10 

3-5 

8.5 

1200 

4 

0-3944 

—  O.OOOI 

7 

0.3945 

.17 

10 

4 

6 

1080 

5 

0.3946 

+  0.0001 

Rate  of  Precipitation. — In  a  solution  of  copper  sulphate 
(5  c.c.  in  volume  and  containing  0.3945  gram  of  metallic 
copper)  slightly  acidulated  with  sulphuric  acid  a  current  of 
5  amperes  and  6  volts  precipitated  the  metal  as  follows : 

In  i  minute   0.1800  gram 

In  2  minutes 0.3400  gram 

In  3  minutes 0.3664  gram 

In  4  minutes 0.3945  gram 

In  5  minutes 0.3945  gram 

Remarks. — The  following  experiment  was  made  to  deter- 
mine what  loss,  if  any,  was  suffered  by  the  mercury  while 
standing  in  the  desiccator.  A  cell  filled  and  prepared  as 
above  was  weighed.  It  was  then  returned  to  the  desiccator 


80  ELECTRO-ANALYSIS. 

and  reweighed  at  intervals  of  twenty-four  hours.  A  loss 
of  o.oooi  gram  per  day  was  observed  during  the  first  week. 
The  rate  of  loss  then  decreased  to  such  an  extent  that  the 
total  loss  after  a  period  of  twenty-six  days  amounted  to 
only  0.0015  gram.  It  was  frequently  found  upon  reweigh- 
ing  a  cell  in  the  morning  that  no  loss  had  occurred,  the  cell 
having  remained  in  the  desiccator  over  night. 

It  is  necessary  to  keep  the  inside  of  the  cell  absolutely 
clean,  otherwise  the  amalgam  shows  a  tendency  to  cling  to 
the  glass.  Losses  may  occur  from  this  source,  as  exceed- 
ingly small  globules  of  mercury  are  often  detached  by  the 
wash  water,  as  well  as  by  the  alcohol  and  ether. 

An  interesting  experiment  that  students  should  perform 
consists  in  dissolving  a  weighed  amount  of  pure  copper  sul- 
phate in  a  small  volume  of  water  (5  to  10  cubic  centimeters) 
and  electrolyzing  the  solution  in  the  manner  just  outlined 
with  a  mercury  cathode  and  a  rotating  anode.  Do  not  add 
any  sulphuric  acid.  When  the  solution  is  colorless  care- 
fully siphon  out  the  acid  liquid  into  a  beaker.  Wash  the 
amalgam  as  before,  combining  the  wash  water  and  the  liquid 
first  removed,  after  which  titrate  this  solution  with  a  TO  nor- 
mal sodium  carbonate  solution.  The  sulphuric  acid  con- 
tent of  the  salt  is  thus  obtained  with  great  accuracy.  The 
increase  in  weight  of  the  mercury  cup  naturally  gives  the 
copper  so  that  a  complete  analysis  of  the  salt  (water  of  crys- 
tallization excepted)  may  be  executed  in  a  very  few  minutes. 

A  metallic  nitrate  may  be  analyzed  as  under  Nitric  Acid, 
p.  289. 

For  the  estimation  of  the  halogen  content  of  metallic 
halides  see  p.  89. 


DETERMINATION    OF    METALS CADMIUM.  8  I 


CADMIUM. 

LITERATURE. — Ber.,  11,2048;  Smith,  Am.  Phil.  Soc.  Pr.,  1878;  Clarke, 
Z.  f.  a.  Ch.,  18,  104;  Beil  stein  and  Jawein,  Ber.,  12,  759;  Smith, 
Am.  Ch.  Jr.,  2,  42;  Luckow,  Z.  f.  a.  Ch.,  19,  16  ;  Wright  son,  Z.  f.  a. 
Ch.,  15,  303;  Classen  and  v.  Reiss,  Ber.,  14,  1628;  Warwick,  Z.  f. 
anorg.  Ch.,  i,  258;  Moore,  Ch.  News,  53,  209  ;  Smith,  Am.  Ch.  Jr.,  12, 
329;  Vortmann,  Ber.,  24,  2749;  Riidorff,  Z.  f.  ang.  Ch.,  Jahrg.  1892; 
Classen,  Ber.,  27,  2060;  Heidenreich,  Ber.,  29,  1586;  Wallace  and 
Smith,  J.  Am.  Ch.  S.,  19,  870  ;  ibid.,  20,  279  ;  Balachowsky ,  C.  r.,  131, 
384;  Miller  and  Page,  Z.  f.  anorg.  Ch.,  28,  233;  Kollock,  J.  Am.  Ch. 
S.,  21,  911  ;  A  very  and  Dales,  J.  Am.  Ch.  S.,  19,  380  ;  M  ed  way ,  Am. 
Jr.  Science  [4th  series],  18,  56;  Flora,  Am.  Jr.  Science  [4th  series], 
20,  268;  Z.  f.  anorg.  Ch.,  47,  13;  Danneel  and  Nissenson,  Internation- 
aler  Congress  fur  angw.  Ch.,  (1903)  Bd.  4,  680;  Exner,  J.  Am.  Ch. 
S.,  25,  902;  Diavison,  J.  Ani.  Ch.  S.,  27,  1275;  Kollock  and  Smith, 
J.  Am.  Ch.  S.,  27,  1528;  Fischer  and  Boddaert,  Z.  f.  Elektrochem., 
10,  948;  Foerster,  Z.  f.  ang.  Ch.,  19,  1890;  Kollock  and  Smith, 
Am.  Phil.  Soc.  Pr.,  45,  260. 

Cadmium  can  be  determined  electrolytically  as  readily 
as  copper.  Prepare  a  solution  of  the  chloride  or  sulphate 
of  definite  strength.  Remove  50  c.c.  to  a  suitable,  weighed 
platinum  vessel.  Add  one  gram  of  pure  potassium  cyanide; 
dilute  with  water  to  125  c.c.,  heat  to  60°,  and  electrolyze 
with  N.D100  =  0.06  ampere  and  3.2  volts.  The  metal  will 
be  completely  deposited  in  five  hours,  or  the  decomposition 
may  be  begun  in  the  evening  and  by  morning  the  metal  will 
be  fully  precipitated.  To  ascertain  whether  the  precipita- 
tion is  complete,  raise  the  level  of  the  liquid  in  the  platinum 
dish.  In  washing,  it  will  not  be  necessary  to  siphon  off  the 
supernatant  liquid;  it  can  be  poured  off,  after  interruption 
of  the  current,  without  loss  of  metal  from  re-solution. 
Wash  the  deposit  with  cold  and  hot  water;  also  with  alco- 
hol and  ether.  Dry  upon  a  warm  iron  plate  (temperature 
not  exceeding  100°  C.). 

This  metal  can  be  deposited  from  the  solution  of  its  phos- 


82 


ELECTRO-ANALYSIS. 


phate  in  phosphoric  acid.  The  conditions  that  follow  gave 
very  satisfactory  results;  a  current  of  N.D100  =  o.o6 
ampere  and  3-7  volts  acted  upon  0.1656  gram  of  cadmium 
as  sulphate,  30  c.c.  of  sodium  phosphate  (1.0358  sp.  gr.), 
and  iy2  c.c.  of  phosphoric  acid  (sp.  gr.  1.347).  The  total 
dilution  equaled  100  c.c.  The  temperature  of  the  solution 
was  50°.  The  precipitated  cadmium  weighed  (a)  0.1654 
gram  and  (b)  0.1657  gram.  The  current  for  the  last  hour 
of  the  decomposition  should  be  increased  and  the  deposit  be 
washed  before  breaking  the  current. 

Cadmium  may  also  be  precipitated  from  a  solution  of  its 
sulphate  containing  a  small  amount  of  free  sulphuric  acid 
(2  c.c.  H2SO4,  sp.  gr.  1.09  for  o.i  gram  of  cadmium). 
Heat  to  50°  and  electrolyze  with  N.D100  =  o.i5  ampere 
and  2.5  volts.  Siphon  off  the  acid  liquid  before  interrupting 
the  current.  Treat  the  deposit  as  previously  directed. 

Cadmium  can  also  be  deposited  quite  readily,  and  in  a 
crystalline  form,  from  its  acetate  solution.  An  example  will 
indicate  the  proper  conditions  for  a  successful  determina- 
tion :  0.1329  gram  of  cadmium  oxide  was  dissolved  in  acetic 
acid,  the  solution  was  evaporated  to  dryness,  and  the  residue 
dissolved  in  30  c.c.  of  water.  The  liquid  was  then  heated 
to  50°  and  electrolyzed  with  a  current  of  0.02  ampere  for 
37  sq.  cm.  of  cathode  surface  and  a  pressure  of  3.5  volts. 
The  metal  was  completely  precipitated  in  four  hours.  It 
was  crystalline  and  adherent.  The  acid  liquid  should  be 
siphoned  off  without  interrupting  the  current.  Good  results 
can  be  obtained  and  the  period  of  precipitation  be  reduced 
by  adding  i  gram  of  ammonium  acetate  to  the  solution  after 
the  current  has  acted  for  an  hour.  When  the  precipitation 
is  completed,  detach  the  dish,  wash  the  deposited  metal  first 
with  warm  water,  then  with  absolute  alcohol,  and  finally 
with  ether.  Dry  upon  a  moderately  warm  plate. 


DETERMINATION    OF    METALS CADMIUM.  83 

Balachowsky,  in  precipitating  cadmium,  makes  use  of  a 
silver-coated  platinum  dish.  Dissolve  from  1.5  to  2  grams 
of  cadmium  sulphate  in  100  c.c.  of  water,  add  5  c.c.  of 
acetic  acid  for  every  gram  of  salt,  heat  to  60°  and  electrolyze 
with  a  current  of  0.004  ampere  per  sq.  cm.  and  2.8  volts. 
Later  increase  the  current  to  0.006  ampere  and  3.5  volts. 
The  deposited  metal  should  be  treated  as  already  described. 

The  same  chemist  also  obtained  very  satisfactory  results 
by  adding  formaldehyde,  acetaldehyde,  or  urea  to  the  solu- 
tion of  cadmium  sulphate.  The  liquid  was  then  heated  to 
60°  and  electrolyzed  with  a  current  of  2.5-3.3  v°lts  and 
0.003  to  0.006  ampere  per  sq.  cm. 

If  desired,  the  metal  can  also  be  precipitated  from  the 
solution  of  the  double  oxalate  of  ammonium  and  cadmium 
(see  Copper),  or  from  a  formate  solution  in  the  presence  of 
free  formic  acid. 

When  using  the  oxalate  solution,  add  to  it  for  every  0.3 
to  0.4  gram  of  sulphate,  10  grams  of  ammonium  oxalate, 
dilute  to  1 20  c.c.  with  water,  heat  to  75°,  and  electrolyze 
with  N.D100  =  0.5-1.5  amperes  and  3-3.5  volts.  The  time 
necessary  for  complete  precipitation  will  be  three  and  one- 
half  hours. 

Avery  and  Dales  employed  the  formate  solution.  Their 
recommendation  is :  Add  6  c.c.  of  formic  acid  (sp.  gr.  1.20) 
to  the  solution  of  cadmium  sulphate,  then  potassium  car- 
bonate until  a  slight  permanent  precipitate  is  formed,  which 
is  just  dissolved  in  formic  acid,  after  which  i  c.c.  of  the  same 
acid  is  introduced,  the  liquid  diluted  to  150  c.c.  and  electro- 
lyzed with  N.D100  =  0.15-0.20  ampere  and  2.6-3.4  volts. 

Vortmann  has  determined  several  metals  quite  satis- 
factorily in  the  form  of  amalgams.  In  applying  his  recom- 
mendation to  cadmium,  add  to  the  solution  of  its  salt  a 
solution  of  mercuric  chloride  and  5  grams  of  ammonium 


84  ELECTRO-ANALYSIS. 

oxalate.  Effect  the  solution  of  the  latter  salt  without  the 
aid  of  heat.  This  procedure  is  only  good  when  small 
amounts  of  cadmium  are  present;  cadmium  ammonium 
oxalate  is  not  very  soluble.  The  current  employed  for  the 
precipitation  should  at  the  very  beginning  of  the  decompo- 
sition equal  from  0.6  to  0.8  ampere.  When  the  amalgam 
of  mercury  and  cadmium  commences  to  separate  reduce  the 
current  to  0.3  ampere,  but  gradually  increase  it  until  at  the 
end  of  the  decomposition  it  has  its  initial  strength.  If  the 
quantity  of  cadmium  exceeds  0.3  gram,  let  the  solution 
undergoing  electrolysis  be  ammoniacal.  To  this  end  add  tar- 
taric  acid  (3  grams)  and  an  excess  of  ammonia  to  the  liquid 
containing  the  mercury  and  the  cadmium.  Dilute  to  200 
c.c.  with  water.  Allow  the  current  to  act  until  a  portion  of 
the  liquid  remains  clear  when  tested  with  ammonium  sul- 
phide. 

In  the  usual  course  of  gravimetric  analysis  cadmium  is 
obtained  as  sulphide.  To  prepare  it  for  electrolysis  dissolve 
the  same  in  nitric  acid,  and  after  expelling  the  excess  of  the 
latter,  add  a  small  amount  of  potassium  hydroxide  (suffi- 
cient to  precipitate  the  cadmium),  and  follow  this  with  an 
excess  of  potassium  cyanide  (i  to  2  grams).  Proceed  fur- 
ther as  already  directed. 

The  Rapid  Precipitation  of  Cadmium  With  the  Use  of 
a  Rotating  Anode. 

Arrange  apparatus  as  outlined  under  COPPER.  To  the 
solution  of  cadmium  sulphate  (  =  0.2756  gram  of  cad- 
mium), add  3  c.c.  of  sulphuric  acid  (i :  10),  dilute  to  125 
c.c.  with  water,  heat  to  incipient  boiling,  remove  the  lamp, 
rotate  the  anode  at  the  rate  of  600  revolutions  per  minute 
and  electrolyze  with  a  current  of  N.D100  =  5  amperes  and 


DETERMINATION    OF    METALS CADMIUM.  85 

8  to  9  volts.  In  ten  minutes  the  precipitation  of  cadmium 
will  be  complete.  In  one  actual  experiment  0.2756  gram 
was  found,  and  in  another  where  0.5512  gram  metal  was 
present  0.5508  gram  was  precipitated  in  fifteen  minutes. 
The  deposits  are  grey  in  color,  crystalline  and  adherent. 
Much  sulphuric  acid  retards  the  complete  deposition  of 
metal.  It  was  also  found  in  the  presence  of  0.5  c.c.  sul- 
phuric acid  (1:10)  by  using  a  current  of  N.D100  — 4 
amperes  and  14  volts  that  as  much  as  0.5762  gram  of  metal 
could  be  precipitated  in  eight  minutes. 
Rate  of  precipitation: 

In  i  minute o.i  190  gram 

In  2  minutes 0.2245  gram 

In  3  minutes 0.3417  gram 

In  5  minutes 0.5217  gram 

In  7 Y2  minutes 0.5760  gram 

In  8  minutes 0.5762  gram 

The  deposition  of  cadmium  from  an  ammoniaeal  electro- 
lyte with  stationary  electrodes  never  gave  satisfaction.  By 
using  a  rotating  anode,  however,  this  electrolyte  may  be 
employed.  To  the  solution  of  the  cadmium  salt  add  ammo- 
nium hydroxide  sufficient  to  precipitate  the  metallic  hydrox- 
ide and  to  redissolve  it.  To  this  solution  add  a  solution  of 
10  c.c.  sulphuric  acid  (1:10)  neutralized  with  ammonia, 
dilute  to  125  c.c.  and  electrolyze  with  N.D100  =  5  amperes 
and  6l/2  volts.  In  ten  minutes  the  deposition  will  be  com- 
plete. In  this  electrolyte  the  rate  of  precipitation  was  as 
follows : 

In  i  minute 0.1312  gram 

In  2  minutes 0.2708  gram 

In  3  minutes 0.2868  gram 

In  4  minutes 0.2889  gram 

In  5  minutes 0.2887  gram 


86  ELECTRO-ANALYSIS. 

As  observed  in  a  preceding  paragraph  a  formate  electro- 
lyte answers  well  for  the  precipitation  of  cadmium.  Upon 
introducing  the  rotating  anode  in  connection  with  it  the 
cadmium  is  deposited  in  a  very  few  minutes.  This  is  evi- 
denced by  one  from  a  number  of  examples : 

To  a  solution,  containing  0.2898  gram  of  cadmium  as 
sulphate  add  five  grams  of  sodium  carbonate  and  16  c.c.  of 
formic  acid  (sp.  gr.  1.06),  after  which  dilute  to  125  c.c., 
heat  the  electrolyte  to  boiling,  remove  the  flame,  rotate  the 
anode  at  600  revolutions  per  minute,  and  apply  a  current  of 
N.D100  =  5  amperes  and  5  volts.  In  fifteen  minutes  0.2900 
gram  of  metal  was  precipitated. 

Again — to  a  solution  containing  0.2898  gram  of  cadmium 
add  1.25  gram  of  sodium  carbonate,  5  c.c.  of  formic  acid 
(sp.  gr.  i. 06)  and  electrolyze  with  N.D100  =  5  amperes 
and  9  volts,  when  the  entire  quantity  of  metal  will  be  pre- 
cipitated in  five  minutes.  Thus  from  this  electrolyte  there 
was  deposited. 

In   i   minute 0.1645  gram  of  cadmium 

In  2   minutes 0.2816  gram  of  cadmium 

In  3   minutes 0.2891   gram  of  cadmium 

In  4  minutes 0.2896  gram  of  cadmium 

In  an  electrolyte  containing  ammonium  formate  in  the 
presence  of  either  ammonium  hydroxide  or  formic  acid  the 
deposition  of  cadmium  takes  place  equally  well.  Thus, 
with  0.2898  gram  of  metal  in  the  presence  of  5  c.c.  of 
ammonium  hydroxide,  -  and  10  c.c.  of  formic  acid  (sp.  gr. 
i. 06)  a  current  of  N.D100  =  5  amperes  and  6  volts,  the 
anode  making  690  revolutions  per  minute,  there  was 
precipitated : 

In  i  minute ..  .0.1612  gram 

In  2  minutes 0.2850  gram 

In  3  minutes 0.2904  gram 


DETERMINATION    OF    METALS CADMIUM.  8/ 

The  deposits  of  metal  resembled  those   from  the  sodium 
formate  electrolyte. 

One  of  the  very  first  electrolytes  suggested  for  the  precip- 
itation of  cadmium  was  sodium  acetate  in  the  presence  of 
free  acetic  acid.  The  results  from  it  have  been  most  satis- 
factory. By  employing  the  rotating  anode  the  time  factor 
may  be  reduced  to  a  few  minutes.  Starting  with  a  cadmium 
sulphate  solution  containing  0.3984  gram  of  metal  add  to 
it  3  grams  of  sodium  acetate  and  0.25  c.c.  of  dilute  acetic 
acid,  dilute  to  125  c.c.  and  electrolyze  with  a  current  of 
N.D100  =  5  amperes  and  8.5  to  9  volts.  The  anode  should 
perform  600  revolutions  per  minute.  With  these  conditions 
the  rate  of  precipitation  will  be 

In  i   minute 0.1601  gram  of  cadmium 

In  2   minutes 0.2863  gram  of  cadmium 

In  3   minutes 0.3963  gram  of  cadmium 

In  4  minutes 0.3987  gram  of  cadmium 

Ammonium  acetate  may  be  substituted  for  the  sodium 
salt.  In  such  cases  it  is  advisable  to  have  acetic  acid  present 
from  the  very  beginning. 

With  an  alkaline  cyanide  electrolyte  follow  the  conditions 
of  an  actual  experiment :  Add  to  a  solution  of  cadmium 
sulphate  (  =  0.4568  gram  of  metal) ,  3  grams  of  pure  potas- 
sium cyanide,  i  gram  of  sodium  hydroxide,  dilute  to  125 
c.c.  with  water  and  electrolyze  with  N.D100  =  5  amperes 
and  5.5  volts.  The  rate  of  precipitation  will  then  be 

In    i  minute 0.1808  gram  of  metal 

In    2  minutes 0.2585  gram  of  metal 

In    3  minutes 0.3291  gram  of  metal 

In    5  minutes 0.3778  gram  of  metal 

In    7}/2  minutes 0.4348  gram  of  metal 

In  i o  minutes 0.4534  gram  of  metal 

In  15  minutes 0.4568  gram  of  metal 


88  ELECTRO-ANALYSIS. 

The  cadmium  deposits  were  here  lustrous  and  of  a  silver- 
white  color. 

Ammonium  and  sodium  acetates  are  not  very  good  elec- 
trolytes for  this  metal,  while  ammonium  succinate  in  the 
presence  of  a  slight  excess  of  succinic  acid  yielded  good  re- 
sults, the  deposits  being  similar  to  those  from  a  formate  or 
an  acetate  electrolyte.  With  sodium  succinate  free  acid  is 
not  favorable  to  the  character  of  the  deposit.  As  much  as 
0.4  gram  of  metal  can  be  deposited  in  a  period  of  ten 
minutes. 

The  Rapid  Precipitation  of  Cadmium  With  the  Use  of 
the  Rotating  Anode  and  Mercury  Cathode. 

Use  the  apparatus  described  under  COPPER  (p.  77). 
Weigh  the  cup  with  its  layer  of  mercury,  introduce  an 
aqueous  solution  of  cadmium  sulphate  (  =  0.9480  gram  of 
metal),  and  apply  a  current  of  1.5  to  3.5  amperes  and  10  to 
7  volts.  At  the  expiration  of  fifteen  minutes  the  precipita- 
tion of  the  cadmium  will  be  finished.  Wash  and  dry  as 
directed  under  COPPER.  The  anode  should  make  360  revo- 
lutions per  minute.  The  amalgam  will  be  quite  bright  in 
appearance.  The  rate  of  precipitation  of  the  cadmium  is  as 
follows : 

In    i  minute 0.1531  gram 

In    2  minutes 0.4984  gram 

In    7  minutes 0.8707  gram 

In    9  minutes 0.9480  gram 

In  i  o  minutes 0.9484  gram 

One  cubic  centimeter  (40  drops)  of  concentrated  sul- 
phuric acid  will  retard  the  deposition  of  this  metal  quite 
markedly.  Half  of  this  volume  of  acid  will  do  no  harm. 

Under  the  preceding  metal,  COPPER,  mention  was  made  of 
the  mercury  cathode  and  the  rotating  anode  in  the  analysis 


DETERMINATION    OF    METALS MERCURY.  89 

of  metallic  sulphates  and  nitrates.  How  the  halogens  may 
be  simultaneously  determined  will  be  outlined  later  (p.  285). 
At  this  point,  however,  it  seems  advisable  to  indicate  the 
course  of  procedure  in  the  analysis  of  a  metallic  halide  when 
the  determination  of  the  halogen  element  is  of  secondary 
importance  while  that  of  the  metal  is  of  chief  importance. 
Using  the  apparatus,  just  employed  with  the  sulphate,  with 
halides,  there  will  under  the  influence  of  high  current  densi- 
ties be  a  copious  evolution  of  halogens  and  these  will  attack 
the  rotating  anode  most  energetically.  To  offset  these  un- 
favorable conditions  place  a  layer  of  toluene  or  xylene  upon 
the  solution  of  the  metal  halide.  Either  liquid  will  com- 
pletely absorb  the  liberated  halogen.  Chlorides  of  cobalt, 
gold,  iron,  mercury  and  tin  were  quickly  analyzed  in  this 
way  with  the  utmost  ease  and  satisfaction.  In  the  case  of 
cadmium  the  bromide  was  used.  Its  solution  was  so  pre- 
pared that  5  c.c.  of  it  contained  0.2212  gram  of  metal. 
After  the  addition  of  10  c.c.  of  toluene  the  liquid  was  elec- 
trolyzed  with  a  current  of  2  amperes  and  5  volts.  The 
toluene  became  red  in  color  but  later  changed  to  yellow. 
The  odor  of  bromine  was  not  detected.  In  ten  minutes 
0.2215  §"ram  of  metal  was  precipitated. 

See  also  J.  Am.  Ch.  S.,  27,   1547,  and  Journal  of  the 
Chemical  Society  (London),  87,  1034. 

MERCURY. 

LITERATURE. — Ber.,  6,  270;  Clarke,  Am.  Jr.  Sc.  and  Ar.,  16,  200; 
Classen  and  L u d w i g ,  Ber.,  19,  323  ;  Hoskinson,  Am.  Ch.  Jr.,  8,  209  ; 
Smith  and  Kn  er  r ,  ibid.,  8,  206  ;  Smith  and  Fr  ankel,  Am.  Ch.  Jr.,  n, 
264;  Smith,  Jr.  An.  Ch.,  5,  202;  Vortmann,  Ber.,  24,  2749;  Brandt. 
Z.  f.  a.  Ch.,  1891,  p.  202;  Riidorff,  Z.  f.  ang.  Ch.,  1892,  p.  5;  Eisen- 
berg,  Thesis,  Heidelberg,  1895;  Schmucker,  J.  Am.  Ch.  S.,  15, 
204;  Fr  ankel,  Jr.  Fr.  Ins.,  1891;  Rising  and  Lenher,  Berg-Hiitt.  Z.. 
55,  J  75  ;  Wallace  and  Smith,  J.  Am.  Ch.  S.,  18,  169  ;  F  e  r  n  b  e  r  g  e  r  and 

9 


9O  ELECTRO-ANALYSIS. 

Smith,  J.  Am.  Ch.  S.,  21,  1006;  Kollock,  J.  Am.  Ch.  S.,  21,  911; 
Bindschedler,  Z.  f.  Elektrochem.,  8,  329;  Glaser,  Z.  f.  Elektrochem., 
9,  ii  ;  Matolcsy,  Ch.  Blatt.,  77  Jahrg.  (1906),  166  ;  Exner,  J.  Am.  Ch. 
S.,  25,  901;  Kollock  and  Smith,  J.  Am.  Ch.  S.,  27,  1537;  R.  O. 
Smith,  J.  Am.  Ch.  S.,  27,  1270;  Fischer  and  Boddaert,  Z.  f. 
Elektrochem.,  10,  949. 

In  preparing  solutions  for  experimental  purposes,  use 
either  mercuric  nitrate  or  chloride.  To  a  definite  portion 
of  such  a  solution  add  3  c.c.  of  concentrated  nitric  acid, 
dilute  to  125  c.c.,  heat  to  70°,  and  electrolyze  with  a  cur- 
rent of  N.D100  =  0.06  ampere  and  2  volts.  The  metal  will 
be  fully  precipitated  in  four  hours.  The  deposit  will  be 
drop-like  in  appearance.  The  acid  liquid  must  be  re- 
moved before  the  interruption  of  the  current  occurs,  or 
sodium  acetate  should  be  added;  then  the  liquid  can  be 
decanted  without  the  possibility  of  loss  from  resolution  of 
the  mercury  (Rudorff). 

A  mercuric  chloride  solution,  feebly  acidulated  with  sul- 
phuric acid  (0.5  c.c.  of  sulphuric  acid),  diluted  to  125  c.c.. 
heated  to  65°,  and  electrolyzed  with  a  current  of  N.D100  = 
0.4-0.6  ampere  and  3.5  volts,  will  yield  all  its  metal  in 
one  hour.  Always  wash  the  deposited  metal  with  cold 
water.  Rudorff  recommended  the  addition  of  the  follow- 
ing substances  to  the  liquid  containing  the  mercury  salt: 
0.5  gram  of  tartaric  acid  and  10  c.c.  of  ammonium  hydrox- 
ide (sp.  gr.  0.91),  or  5  c.c.  of  nitric  acid,  10  c.c.  of  a 
saturated  solution  of  sodium  pyrophosphate,  and  10  c.c.  of 
ammonium  hydroxide.  A  current  of  0.02  ampere  will  pre- 
cipitate the  mercury  in  a  compact,  adherent  form. 

From  experiments  made  in  this  laboratory  the  writer 
prefers  and  would  especially  recommend  solutions  of  the 
double  cyanide  of  mercury  and  potassium  for  the  electro- 
lytic deposition  of  mercury.  To  the  mercury  salt  solu- 


DETERMINATION    OF    METALS MERCURY.  9! 

tion  add  I  gram  of  pure  potassium  cyanide  for  every  o.i- 
0.2  gram  of  metal,  dilute  with  water  to  100  c.c.,  heat  to 
65°,  and  electrolyze  with  a  current. of  N.D100  =  0.02-0.07 
ampere  and  1.6-3.2  volts.  As  much  as  0.25  gram  of  metal 
can  be  deposited  in  three  hours.  This  procedure  requires 
no  further  attention  after  it  is  once  set  in  operation.  The 
deposit  is  always  compact,  and  gray  in  color.  Use  water 
only  in  washing  it,  for  alcohol  seems  to  detach  some  of  the 
metallic  film.  In  all  precipitations  of  mercury  it  is  advis- 
able to  have  this  metal  deposited  upon  a  layer  of  metallic 
silver,  hence  invariably  coat  the  platinum  dishes  with  this 
metal. 

Classen  recommends  the  double  oxalate  solution  for 
electrolytic  purposes,  and  to  that  end  adds  to  the  mercuric 
chloride  solution  from  4  to  5  grams  of  ammonium  oxalate, 
dilutes  with  water  to  120  c.c.,  and  electrolyzes  at  29-37° 
with  a  current  of  N.D100  =  i  ampere  and  4.05-4.7  volts. 
The  mercury  comes  down  in  a  perfectly  adherent  form, 
the  time  depending  entirely  upon  the  pressure. 

The  precipitation  is  also  very  satisfactory  in  a  phosphoric 
acid  solution,  as  is  seen  in  the  following  example :  To  a 
solution,  containing  0.1159  gram  of  mercury,  were  added 
30  c.c.  of  sodium  phosphate  (sp.  gr.  1.038)  and  5  c.c.  of 
phosphoric  acid  (sp.  gr.  1.347),  after  which  it  was  diluted 
to  175  c.c.  with  water,  heated  to  50°,  and  electrolyzed  for 
four  hours  with  a  current  of  N.D100  =  o.O4  ampere  and 
1.6  volts.  The  deposit  of  mercury  weighed  0.1162  gram. 
It  was  treated  in  the  usual  manner. 

In  general  analysis  mercury  is  frequently  obtained  as 
sulphide.  Its  determination  in  this  form  requires  time  and 
exceeding  care.  It  is,  however,  soluble  in  the  fixed  alkaline 
sulphides  containing  free  alkali.  The  writer  has  discovered 


92  ELECTRO- ANALYSIS. 

that  such  a  solution  can  be  electrolyzed  without  difficulty; 
the  mercury  is  deposited  from  it  in  a  very  compact  form. 
An  actual  analysis  conducted  in  this  laboratory  will  best 
present  the  proper  conditions  for  a  successful  determina- 
tion: 20  c.c.  of  a  sodium  sulphide  solution  (sp.  gr.  1.19) 
were  added  to  a  mercuric  chloride  solution  (=  0.1903  gram 
of  mercury),  and  the  whole  then  diluted  to  125  c.c.  with 
water.  This  was  acted  upon  with  a  current  of  N.D100  = 
o.i  i  ampere  and  2.5  volts  for  five  hours.  The  temperature 
of  the  solution  was  70°.  The  weight  of  the  precipitated 
mercury  was  0.1902  gram.  It  was  further  treated  as  ad- 
vised in  the  preceding  paragraphs.  It  is  best  to  use  a  plati- 
num dish  as  the  negative  electrode  and  a  platinum  spiral 
(p.  73)  for  the  anode.  Dry  the  deposit  on  a  moderately 
warm  plate  or  over  sulphuric  acid. 

Several  determinations  of  mercury  in  cinnabar  were 
made  to  test  the  general  applicability  of  the  method. 
Samples  of  the  mineral,  analyzed  in  the  usual  gravimetric 
way,  showed  the  presence  of  85.40  per  cent,  of  metallic 
mercury.  Portions  of  the  same  mineral  were  weighed  out 
in  platinum  dishes  and  after  solution  in  20  to  25  c.c.  of 
sodium  sulphide  of  the  specific  gravity  previously  men- 
tioned, were  diluted  with  water  to  125  c.c.  and  electrolyzed 
at  70°,  with  the  conditions  recorded  in  the  preceding  para- 
graph. The  period  of  time  allowed  for  the  precipitations 
never  exceeded  three  hours.  The  results  were:— 

CINNABAR,  IN  MERCURY,  IN  MERCURY 

GRAMS.                          GRAMS.  PERCENTAGE. 

0.2167                        0.1850  85.37 

0.2432                        0.2077  85.40 

The  platinum  dishes  were  covered  during  the  electrolytic 
decomposition.  It  should  be  done  in  the  determination 
of  every  metal.  Its  purpose  here  was  to  prevent  evapora- 


DETERMINATION    OF    METALS MERCURY.  93 

tion,  thereby  exposing  a  rim  of  metal,  which,  if  in  part  not 
volatilized,  would  yet  be  changed  to  mercury  sulphide,  indi- 
cated by  a  dark-colored  film. 

The  Rapid  Precipitation  of  Mercury  With  the  Use  of 
a  Rotating  Anode. 

In  a  nitric  acid  electrolyte  with  0.5840  gram  of  mercury 
as  mercurous  nitrate  and  one  cubic  centimeter  of  concen- 
trated nitric  acid,  a  current  of  N.D100  =  7  amperes  and  12 
volts  precipitated  the  whole  of  the  metal  in  seven  minutes. 
The  anode  performed  700  revolutions  per  minute. 

To  show  the  rate  of  precipitation  from  this  electrolyte 
a  solution  containing  0.5120  gram  of  metal  was  exposed 
to  the  action  of  the  current  with  the  following  results : 

Metal  deposited  in     2  minutes 0.3612  gram 

Metal  deposited  in     4  minutes 0.4772  gram 

Metal  deposited  in     8  minutes 0.5077  gram 

Metal  deposited  in   10  minutes 0.5122  gram 

Metal  deposited  in   12  minutes 0.5121  gram 

Metal  deposited  in  20  minutes 0.5119  gram 

In  these  speed  trials  the  pressure  never  exceeded  7  volts. 
It  was  usually  6.5  volts.  The  total  dilution  of  the  electro- 
lyte was  115  cubic  centimeters. 

Upon  using  an  alkaline  sulphide  electrolyte  it  was  found 
to  answer  admirably  in  the  precipitation  of  mercury  with 
the  help  of  a  rotating  anode.  Thus  to  a  mercuric  chloride 
solution,  containing  0.2603  gram  of  metal,  were  added  10 
c.c.  of  a  sodium  sulphide  solution  of  sp.  gr.  1.17,  diluted 
to  115  c.c.,  and  electrolyzed  with  a  current  of  N.D100  =  6 
amperes  and  7  volts,  the  anode  being  rotated  as  indicated 
in  the  preceding  paragraph.  In  fifteen  minutes  0.2602 
gram  of  metal  was  precipitated. 


94  ELECTRO-ANALYSIS. 

The  rate  of  precipitation  was  found  to  be : 

Metal  deposited  in     2  minutes 0.1371  gram 

Metal  deposited  in     5  minutes 0.2198  gram 

Metal  deposited  in     8  minutes 0.2538  gram 

Metal  deposited  in   10  minutes 0.2554  gram 

Metal  deposited  in  12  minutes 0.2596  gram 

Metal  deposited  in   13  minutes 0.2601   gram 

Metal  deposited  in   15  minutes 0.2602  gram 

Metal  deposited  in  20  minutes 0.2604  gram 

This  scheme  may  be  applied  in  determining  the  mercury 
in  cinnabar  as  described  in  an  earlier  paragraph.  For  ex- 
ample, an  ore  that  showed  the  presence  of  46.20  per  cent, 
mercury,  when  analyzed  by  the  distillation  method,  gave 
46.40,  46.46,  46.40,  46.41,  46.40,  46.46  per  cent,  by  the 
procedure  just  outlined.  The  deposits  of  mercury  were  all 
that  could  be  desired.  The  time  necessary  for  each  determi- 
nation, from  the  weighing  of  the  ore  until  the  mercury 
deposit  itself  was  weighed,  did  not  exceed  an  hour  and 
thirty  minutes.  The  quantity  of  ore  varied  from  0.3000 
gram  to  0.5000  gram. 

It  is  not  too  much  to  say  that,  in  the  light  of  many  simi- 
lar experiences  had  in  this  laboratory,  the  electrolytic  method 
is  vastly  superior  to  the  time-honored  methods  generally 
employed  in  the  estimation  of  mercury. 

The  Rapid  Precipitation  of  Mercury  With  the  Use  of 
the  Rotating  Anode  and  Mercury  Cathode. 

Use  the  same  apparatus  here  as  described  under  cadmium 
and  copper.  A  mercurous  nitrate  solution  contained 
0.3570  gram  of  mercury  in  five  cubic  centimeters.  Nitric 
acid,  sufficient  to  prevent  the  formation  of  a  basic  salt,  was 
also  present.  Using  a  current  of  3  amperes  and  a  pressure 
of  7  to  5  volts  the  rate  of  precipitation  was : 


DETERMINATION    OF    METALS BISMUTH.  95 

In  i  minute 0.2777  gram  of  mercury 

In  2  minutes 0.3542  gram  of  mercury 

In  3  minutes 0.3572  gram  of  mercury 

Dilution  with  water  to  25  c.c.  prolonged  the  period  of 
complete  precipitation  to  8  minutes.  The  addition  of  too 
much  free  nitric  acid  also  exerted  a  retarding  influence. 

Mercuric  chloride  may  also  be  analyzed  in  this  way,  ap- 
plying, however,  the  precautionary  method  of  adding 
toluene  (p.  89)  so  that  the  anode  is  not  attacked  by  the 
liberated  chlorine.  Thus,  to  5  c.c.  of  this  salt,  equivalent 
to  0.2525  gram  of  mercury,  were  added  10  c.c.  of  toluene 
and  the  decomposition  made  with  a  current  of  from  i  to  3 
amperes  and  10  to  7.5  volts.  In  ten  minutes  the  metal  was 
completely  deposited. 

Trials  recently  conducted  in  this  laboratory  prove  that 
if  cinnabar  is  decomposed  with  aqua  regia,  the  solution 
evaporated  to  dryness,  the  residue  taken  up  with  water  and 
filtered  from  gangue  the  liquid  may  be  electrolyzed  in  the 
manner  just  described  with  good  results. 


BISMUTH. 

LITERATURE. — Luckow,  Z.  f.  a.  Ch.,  19,  16;  Classen  and  v.  Reiss, 
Ber.,  14,  1622;  Thomas  and  Smith,  Am.  Ch.  Jr.,  5,  114;  Moore,  Ch. 
N.  53,  209;  Smith  and  Knerr,  Am.  Ch.  Jr.,  8,  206;  Schucht,  Z.  f.  a. 
Ch.,  22,  492;  Eliasberg,  Ber.,  19,  326;  Brand,  Z.  f.  a.  Ch.,  28,  596; 
Vortmann,  Ber.,  24,  2749  ;  Riido  r  f.f ,  Z.  f.  ang.  Ch.,  1892,  199  ;  Smith 
and  Saltar,  Z.  f.  anorg.  Ch.,  3,  418;  Smith  and  Moyer,  J.  Am.  Ch.  S., 
15,  28;  ibid.,  15,  1 01  ;  Wieland,  Ber.,  17,  1612;  Smith  and  Knerr, 
Am.  Ch.  Jr.,  8,  206;  Schmucker,  Z.  f.  anorg.  Ch.,  5,  199;  J.  Am.  Ch. 
S.,  15,  203;  Kollock,  J.  Am.  Ch.  S.,  21,  925;  Wimmenauer,  Z.  f. 
anorg.  Ch.,  27,  i;  Brunck,  Ber.,  35,  1871;  Balachowsky,  C.  r.,  131, 
179-182;  Ho  Hard  and  Bertiaux,  C.  r.,  cxxxix  (1904),  839;  Exner. 
J.  Am.  Ch.  S.,  25,  901;  KoUock  and  Smith,  J.  Am.  Ch.  S.,  27,  1539; 
Fischer  and  Boddaert,  Z.  f.  Elektrochern.,  10,  947. 


9  ELECTRO-ANALYSIS. 

The  electrolytic  determination  of  bismuth  has  received 
much  attention.  Numerous  electrolytes  have  been  sug- 
gested. Most  of  them  have  failed  in  that  the  deposits  of 
metal,  unless  very  small  in  amount,  have  almost  invaria- 
bly been  dark  in  color  and  have  shown  a  tendency  to  spongi- 
ness.  Yet  they  were  in  nearly  all  cases  adherent.  There 
has  been  an  additional  objection  in  many  of  the  methods 
to  the  separation  of'  peroxide  upon  the  anode.  In  short, 
the  appearance  of  bismuth  at  both  poles  has  been  very  dis- 
turbing. For  these  reasons  many  of  the  earlier  suggestions 
have  been  abandoned,  and  will  be  omitted  from  the  present 
text. 

Vortmann  prefers  the  amalgam  method,  in  accordance 
with  which  dissolve  0.5  gram  of  bismuth  trioxide  and  2 
grams  of  mercuric  oxide  in  sufficient  nitric  acid  for  the 
purpose,  dilute  with  water  to  150  c.c.,  and  at  the  ordinary 
temperature  electrolyze  with  N.D100  =  i  ampere  and  3.5 
volts.  The  amalgam,  when  the  ratio  is  4Hg  to  iBi,  will 
be  silver-white  in  color.  It  should  be  washed  without  in- 
terrupting the  current,  then  carefully  dried  and  weighed 
The  method  is  said  to  be  especially  well  adapted  for  the 
precipitation  of  large  quantities  of  bismuth. 

Wimmenauer  has  reviewed  the  different  methods  pro- 
posed from  time  to  time,  and  from  his  experience  recom- 
mends the  following  procedure:  Dissolve  0.1-0.3  gram  of 
bismuth  nitrate  in  2-4  c.c.  of  a  glycerol  solution  (i  part 
of  commercial  glycerol  and  2  parts  of  water),  dilute  with 
water  to  150  c.c.,  and  electrolyze  at  50°,  in  a  roughened 
dish,  with  a  current  of  N.D100  =  o.i  ampere  and  2  volts. 
The  anode  is  rotated  during  the  decomposition.  This  can 
be  accomplished  by  a  small  electric  motor,  as  shown  in 
Fig.  26.  The  rotation  is  supposed  to  prevent  the  forma- 
tion of  peroxide,  because  the  latter,  by  the  movement  of 


DETERMINATION    OF    METALS BISMUTH. 


97 


the  anode,  is  immediately  brought  in  contact  with  dilute 
nitric  acid,  in  which  it  dissolves.  When  the  anode  is  at 
rest,  a  protective  layer  of  gas  forms  about  it,  and  this  is 
favorable  to  the  deposition  of  peroxide. 

FIG.  26. 


A.  L.  Kammerer,  who  has  very  recently  made  an  ex- 
haustive study  on  the  electrolytic  determination  of  bis- 
muth in  this  laboratory,  where  he  has  tried  every  form  of 
cathode  and  anode  with  varying  electrolytes,  concludes  that 
the  following  conditions  may  be  relied  upon  to  yield  satis- 
factory results:  0.10-0.15  gram  of  metal  in  i  c.c.  of  nitric 
acid  (sp.  gr.  1.42),  2  c.c.  of  sulphuric  acid  (sp.  gr.  1.84), 
i  gram  of  potassium  sulphate,  150  c.c.  total  dilution 
N.D100  =  o.02  ampere,  V— 1.8.  Temperature,  45°-5o°; 
time,  6-7  hours. 

The  current  should  be  increased  the  last  hour  to  0.15 


10 


98  ELECTRO-ANALYSIS. 

ampere.  Heat  is  absolutely  essential  in  order  to  get  a 
bright  metallic  deposit  of  metal.  The  deposit  should  be 
washed  without  interrupting  the  current,  just  as  has  been 
recommended  with  other  metals  when  precipitated  from 
an  acid  solution.  Close-fitting  cover-glasses  should  always 
be  used  to  reduce  the  evaporation  to  a  minimum.  The 
metal  seemed  to  be  deposited  as  well  upon  smooth  as  upon 
roughened  surfaces. 

The  many  successful  determinations  made  in  accord- 
ance with  the  directions  just  described  indicate  that  the 
method  is  perhaps  the  best  which  has  ever  been  applied  in 
the  case  of  this  particular  metal. 

In  determining  bismuth  Balachowsky  keeps  in  view  the 
following  points:  (a)  A  slightly  acid  solution;  (b)  the 
absence  of  large  amounts  of  the  halogens;  (c)  the  use  of  a 
low  current  density  (not  exceeding  0.06  ampere  per  square 
decimeter)  ;  (d)  a  roughened  dish;  (e)  the  addition  of  urea 
or  aldehyde;  and  offers  this  example:  0.06-1.7  grams  of 
bismuth  sulphate,  5-7  c.c.  of  nitric  acid,  150  c.c.  of  water, 
3.5-5  grams  of  urea;  N.D100  =  0.04-0.06  ampere  and  1-2 
volts.  Temperature,  6o°— 76° ;  time,  6-10  hours. 

When  it  is  necessary  to  use  an  alkaline  citrate  or  citric 
acid  solution  in  the  precipitation  of  bismuth,  observe  the 
following  conditions:  0.1822  gram  of  bismuth,  3  grams  of 
citric  acid,  125  c.c.  total  dilution;  N.D100  =  o.O3  ampere, 
volts  =  2.  Temperature,  65°;  time,  6  hours.  0.1820 
gram  of  bismuth  was  found.  Weigh  the  anode  before  and 
after  the  electrolysis. 

The  Rapid  Precipitation  of  Bismuth  With  the  Use  of 
a  Rotating  Anode. 

As  much  as  0.5510  gram  of  the  metal,  in  the  presence  of 
i  c.c.  of  concentrated  nitric  acid,  may  be  precipitated  in 


DETERMINATION    OF    METALS BISMUTH.  99 

twenty  minutes  with  a  current  of  N.D100  =  i  ampere  and 
2.5  volts.  The  anode  should  rotate  at  the  rate  of  700  to 
900  revolutions  per  minute.  At  first  the  deposit  of  metal 
will  be  white  and  crystalline,  becoming  loose  and  black 
later  but  sufficiently  adherent  for  washing  and  weighing 
purposes. 

It  is  preferable,  however,  to  precipitate  the  bismuth  in 
the  presence  of  mercury  as  an  amalgam.  Thus  to  a  solu- 
tion of  bismuth  nitrate,  equivalent  to  0.2970  gram  of  metal 
add  as  much  mercury  in  the  form  of  mercurous  nitrate 
and  i  c.c.  of  concentrated  nitric  acid.  Heat  the  solution 
to  boiling  and  electrolyze  with  a  current  of  N.D100  =  5 
amperes  and  8.5  volts.  Complete  precipitation  of  the  metals 
as  an  amalgam  will  occur  in  from  eight  to  ten  minutes. 

The  Rapid  Precipitation  of  Bismuth  With  the  Use  of  a 
Rotating  Anode  and  a  Mercury  Cathode. 

Frequent  reference  has  been  made  in  preceding  para- 
graphs concerning  the  difficulty  experienced  in  the  pre- 
cipitation of  the  metal  bismuth  and  emphasis  laid  repeatedly 
on  the  strict  observance  of  the  working  conditions  which 
proved  satisfactory  so  that  naturally  the  analyst  uncon- 
sciously turns  from  the  electrolytic  procedure  when  esti- 
mating this  metal.  However,  with  the  simple  device  of 
a  mercury  cup  and  rotating  anode  as  outlined  and  used  with 
the  preceding  metals  the  determination  can  be  made  with- 
out trouble. 

To  a  solution  of  0.2273  gram  of  metal,  not  exceeding 
12  c.c.  in  volume,  add  0.5  c.c.  of  concentrated  nitric  acid  and 
electrolyze  with  a  current  of  4  amperes  and  5  volts.  All 
the  metal  will  be  precipitated  in  twelve  minutes.  Use  a 
perfectly  smooth  anode.  When  it  is  rough  peroxide,  in 
slight  amount,  may  at  the  beginning  of  the  experiment 


IOO  ELECTRO-ANALYSIS. 

appear  on  it  but  it  will  rapidly  go  away.  The  rotation  of 
the  anode  should  be  quite  rapid,  so  that  the  mercury  may 
take  up  the  bismuth  which  is  deposited  quickly,  as  it  often 
collects  in  a  black  mass  beneath  the  anode. 

The  rate  of  precipitation  from  this  electrolyte  is : 

In    i  minute 0.1305  gram  of  metal 

In    3  minutes 0.2274  gram  of  metal 

In    5  minutes 0.2515  gram  of  metal 

In    8  minutes 0.2732  gram  of  metal 

In  10  minutes 0.2751  gram  of  metal 

In  1 2  minutes . . 0.2775  gram  of  metal 

The  substitution  of  sulphuric  for  nitric  acid  makes  very 
little  difference  in  the  rate  at  which  bismuth  is  precipitated : 

In    5    minutes 0.2409  gram 

In  10   minutes 0.2764  gram 

In  1 5    minutes 0.2770  gram 


LEAD. 

LITERATURE. — Kiliani,  Berg-Hiitt.  Z.,  1883,  253;  Luckow,  Z.  f. 
a.  Ch.,  19,  215;  Riche,  Ann.  de  Chim.  et  de  Phys.  [5  ser.],  13,  508;  Z. 
f.  a.  Ch.,  21,  117  ;  Classen,  ibid.,  257;  Hampe,  Z.  f.  a.  Ch.,  13,  183  ;  May, 
Am.  Jr.  Sc.  and  Ar.  [3  ser.],  6,  255;  also  Z.  f.  a.  Ch.,  14,  347;  Parodi 
and  Mascazzini,  Ber.,  10,  1098;  Z.  f.  a.  Ch.,  16,  469;  18,  588;  Riche, 
Z.  f.  a.  Ch.,  17,  219;  Schucht,  Z.  f.  a.  Ch.,  21,  488;  Tenny,  Am.  Ch. 
Jr.,  5,  413;  Smith,  Am.  Phil.  Soc.  Pr.,  24,  428;  Vortmann,  Ber.,  24, 
2749;  Riidorff,  Z.  f.  ang.  Ch.,  1892,  p.  198;  Warwick,  Z.  f.  anorg.  Ch., 
i,  258;  Classen,  Ber.,  27,  163;  Kreichgauer,  Ber.,  27,  315;  Z.  f. 
anorg.  Ch.,  9,  89;  Classen,  Ber.,  27,  2060;  Medicus,  Ber.,  25,  2490; 
Neumann,  Ch.  Z.  (1896),  20,  381;  Hollard,  B.  s.  Ch.  Paris,  19,  911; 
Linn,  J.  Am.  Ch.  S.,  24,  435;  Marie,  Ch.  Z.,  24,  341,  480;  Nissenson 
and  Neumann,  Ch.  Z.,  19,  1143;  Elbs  and  Rixon,  2.  f.  Elektrochem., 
9,  267;  Danneel  and  Nissenson,  Internationaler  Congress  fur  angew. 
Ch.  (1903),  Band  4,  677;  Hollard,  B.  s.  Ch.,  Series  3,  31,  No.  5;  Ch. 
N.,  89,  278;  Meillere,  J.  Phar.  Chim.,  [6]  16,  465;  Guess,  Eng. 
Min.  Jr.,  81,  328  (1906);  Hollard,  Ch.  Z.,  27,  141  (1903);  Exner,  25, 


DETERMINATION    OF    METALS LEAD.  IOI 

J.  Am.  Ch.  S.,  25,  904;  R.  O.  Smith,  J.  Am.  Ch.  S.,  27,  1287;  Fischer 
and  Boddaert,  Z.  f.  Elektrochem.,  10,  949;  Vortmann,  Ann.,  351,283. 

The  metal  may  be  obtained  by  electrolyzing  solutions 
of  the  double  oxalate  (see  Copper  and  Cadmium),  the 
acetate,  the  oxide  in  sodium  hydroxide,  or  the  phosphate 
dissolved  in  the  latter  reagent  or  in  phosphoric  acid  of  1.7 
specific  gravity.  While  the  metal  separates  well  from 
either  one  of  these  solutions,  difficulty  is  experienced  in 
drying  the  deposit,  for  the  moist  metal  almost  invariably 
suffers  a  partial  oxidation,  thus  rendering  the  results  high. 
The  deposit  can  be  dried,  without  oxidation,  in  an  atmos- 
phere of  hydrogen,  but  for  the  inexperienced  operator 
this  procedure  offers  little  satisfaction.  It  is,  therefore, 
better  to  utilize  the  tendency  of  lead  to  separate,  from 
acid  solutions,  as  the  dioxide.  For  trial  purposes  make 
up  a  definite  volume  of  lead  nitrate.  Electrolyze  several 
portions  (=0.1  gram  lead  each)  in  a  platinum  dish  con- 
nected with  the  anode,  using  a  current  of  N.D100  =  1.5-1.7 
amperes  and  2.36  to  2.41  volts.  The  volume  of  the  elec- 
trolyte should  be  100  c.c.,  and  its  temperature  5O°-6o°.  In 
order  that  the  lead  may  be  precipitated  wholly  as  dioxide 
upon  the  positive  electrode  and  none  in  metallic  form  upon 
the  cathode,  it  is  necessary  that  the  solution  being  analyzed 
should  contain  20  c.c.  of  nitric  acid  of  specific  gravity 
1.35-1.38.  This  quantity  of  acid  is  required  when  lead 
alone  is  present  in  solution.  To  hasten  the  solution  of 
any  metal  which  may  have  found  its  way  to  the  cathode 
interrupt  the  current  for  a  short  time — five  seconds — about 
the  middle  of  the  determination  and  again  for  a  brief  period 
before  the  precipitation  is  finished.  Chlorides  must  be 
absent.  In  the  presence  of  other  metals  the  complete  depo- 
sition of  the  lead  as  dioxide  occurs  with  even  less  acid. 
At  the  end  of  the  precipitation  siphon  off  the  acid  liquid 


102  ELECTRO-ANALYSIS. 

and  wash  in  the  dish,  then  dry  the  deposit  at  i8o°-i9O°  C, 
and  weigh.  The  weight  multiplied  by  0.866  gives  the 
quantity  of  metallic  lead  present.  Numerous  experiments 
made  in  this  laboratory  showed  that  the  deposits  of  lead 
dioxide  will  weigh  too  much  unless  they  have  been  dried 
for  definite  periods  at  a  temperature  ranging  from  200°- 
230°  C.  It  is  not  probable  that  the  excessive  weight  is  due 
to  the  formation  of  a  higher  oxide  than  the  dioxide  but  to 
adherent  and  included  water,  expelled  with  difficulty.  From 
a  series  of  results  made  upon  the  drying  of  the  dioxide  at 
different  temperatures  it  would  seem  as  if  the  factor  with 
which  to  multiply  the  dioxide  should  be  0.8643.  The  de- 
posit can  be  readily  dissolved  in  nitric  acid  to  which  oxalic 
acid  is  added,  or  cover  it  with  dilute  nitric  acid  and  insert 
a  rod  of  zinc  or  copper.  Henz  recommends  a  nitrite  solu- 
tion, acidified  with  nitric  acid,  for  this  purpose.  Reference 
to  the  literature  shows  that  May  preferred,  after  drying  the 
deposit,  to  carefully  ignite  it  and  finally  weigh  as  lead  oxide 
(PbO).  This  precipitation  of  lead  as  dioxide  affords  an 
excellent  method  by  which  to  separate  it  from  other  metals, 
e.  g.}  mercury,  copper,  cadmium,  silver,  and  all  those  solu- 
ble in  nitric  acid,  or  those  which,  in  a  nitric  acid  solution, 
are  deposited  upon  the  cathode. 

Use  in  these  determinations  a  Classen  dish,  the  inner 
surface  of  which  has  been  roughened  by  having  had  a  sand 
blast  projected  against  it.  The  deposition  of  the  dioxide 
will  be  much  accelerated;  e.  g.}  a  few  hours  (4-5)  will  be 
sufficient  for  the  precipitation  of  as  much  as  4  grams  of 
dioxide  upon  100  cm2  surface  with  a  current  of  1.5  am- 
peres. Wash  with  water  and  alcohol,  then  dry  as  pre- 
viously directed. 

The  presence  of  arsenic  in  the  solution  lowers  the  lead 
results.  When  its  quantity  is  very  trifling  the  discrepancy 
may  be  disregarded.  Selenium  has  a  similar  effect. 


DETERMINATION    OF    METALS LEAD.  1 03 

Lead  dioxide,  like  manganese  dioxide  (p.  135),  is  not 
separated  from  solutions  containing  an  excess  of  an  alkaline 
sulphocyanide,  and  if  already  precipitated  as  dioxide,  will 
redissolve  upon  the  addition  of  the  sulphocyanide. 

In  the  analysis  of  lead  ores  Nissenson  and  Neumann 
dissolve  0.5  gram  of  the  material  in  30  c.c.  of  nitric  acid  of 
1.4  specific  gravity,  boil,  dilute  with  water,  filter  into  a 
platinum  dish,  and  electrolyze  at  6o°-7o°  with  a  current 
of  N.D]00— i  ampere  and  2.5  volts.  The  dioxide  is 
washed  and  dried  as  indicated  above.  One  hour  is  suffi- 
cient for  the  precipitation. 

The  suggestion  made  by  Vortmann  that  lead  should  be 
precipitated  as  an  amalgam  is  not  feasible,  owing  to  cer- 
tain difficulties.  His  method,  however,  will  serve  for  the 
separation  of  the  lead  from  a  few  metals. 

The  Rapid  Precipitation  of  Lead  Dioxide  With  the  Use 
of  a  Rotating  Electrode. 

Exner  added  20  c.c.  of  concentrated  nitric  acid  to  a  solu- 
tion of  lead  nitrate,  giving  a  total  volume  of  about  125  c.c. 
and  acted  upon  the  same  with  a  current  of  N.D100  =  10 
amperes  and  4.5  volts.  The  rotating  electrode  (cathode) 
performed  600  revolutions  per  minute.  The  deposits  had 
a  uniform,  velvety  black  color.  There  was  no  tendency 
on  the  part  of  the  deposit  to  scale  off  though  more  than  a 
gram  of  the  dioxide  was  precipitated.  The  time  varied 
from  ten  to  fifteen  minutes.  A  platinum  dish  with  sand- 
blasted inner  surface  was  used  as  anode. 

R.  O.  Smith  in  using  a  current  of  N.D100  =  1 1  amperes 
and  4  volts  upon  a  solution  of  lead  nitrate  containing  0.4996 
gram  of  lead  or  0.5787  gram  of  dioxide — found  the  rate 
of  precipitation  to  be : 


104  ELECTRO-ANALYSIS. 

In    5  minutes 0.4940  gram  lead  dioxide 

In  10  minutes 0.5708  gram  lead  dioxide 

In  15  minutes 0.5747  gram  lead  dioxide 

In  20  minutes 0.5770  gram  lead  dioxide 

In  25  minutes 0.5787  gram  lead  dioxide 

In  30  minutes 0.5789  gram  lead  dioxide 

The  maximum  time  period  for  a  quarter  of  a  gram  of 
metal  is  fifteen  minutes,  and  the  maximum  time  for  a  half- 
gram  of  metal  is  twenty-five  minutes. 


SILVER. 

LITERATURE.-— Luckow,  Ding.  p.  Jr.,  178,  43;  Z.  f.  a.  Ch.,  19,  15; 
Fresenius  and  Bergmann,  Z.  f.  a.  Ch.,  19,  324;  K  rut  wig,  Ber.,  15, 
1267;  Schucht,  Z.  f.  a.  Ch.,  22,  417;  Kinnicutt,  Am.  Ch.  Jr.,  4,  22; 
Rudorff,  Z.  f.  ang.  Ch.,  Jahrg.  1892,  p.  5;  Eisenberg,  Thesis,  Heidel- 
berg, 1895;  Smith,  Am.  Ch.  Jr.,  12,  335;  Fulweiler  and  Smith,  J. 
Am.  Ch.  S.,  23,  583;  Exner,  J.  Am.  Ch.  S.,  25,  900;  Gooch  and 
Medway,  Am.  Jr.  Sciences,  15,  320;  ibid.,  Ch.  N.,  87,  284;  Kollock 
and  Smith,  J.  Am.  Ch.  S.,  27,  1536;  Langness,  J.  Am.  Ch.  S.,  29,  464; 
Fischer  and  Boddaert,  Z.  f.  Elektrochem.,  10,  949. 

The  experiments  of  Luckow  showed  that  this  metal 
could  be  deposited  from  solutions  containing  as  high  as 
eight  to  ten  per  cent,  of  free  nitric  acid.  The  deposit  was 
spongy,  and  there  was  a  simultaneous  deposition  of  silver 
peroxide  at  the  anode.  This  was,  however,  prevented  by 
adding  to  the  solution  some  glycerol,  lactic  or  tartaric  acid. 
A  voluminous  mass  was  also  obtained  from  silver  solutions, 
containing  an  excess  of  ammonium  hydroxide  or  carbonate, 
and  peroxide  appeared  at  the  same  time  upon  the  anode. 

Fresenius  and  Bergmann,  who  have  given  the  electrolysis 
of  acid  solutions  of  silver  particular  study,  observed  that 
the  tendency  of  the  metal  to  sponginess  is  most  marked  when 
the  electrolyte  is  concentrated  and  acted  upon  by  a  strong 
current.  In  a  dilute  liquid,  the  current  being  feeble,  the  de- 


DETERMINATION    OF    METALS SILVER. 


105 


FIG.  27. 


posit  was  compact  and  metallic  in  appearance  (free  acid 
should  be  present).  From  neutral  solutions,  although  very 
dilute,  the  metal  is  separated  in  a  flocculent  condition  by  the 
feeblest  currents.  Therefore,  to  obtain  results  that  would 
answer  for  quantitative  analysis,  the  following  conditions 
were  adopted :  The  total  dilution  of  the  solution  was  200 
c.c. ;  in  this  there  were  0.03-0.04  gram  of  silver,  and  3-6 
grams  of  free  nitric  acid.  The  poles  were  separated  about 
i  cm.  from  each  other,  while  the  current  at  5O°-6o°  was 
N.D100  =  0.04-0.05  ampere, 
and  at  the  ordinary  tempera- 
ture it  was  N.Dj  oo  =  0.1-0.2 
ampere  and  2  volts. 

In  the  experiments  of  Fre- 
senius  and  Bergmann  appa- 
ratus similar  to  that  in  Fig.  27 
was  employed.  It  has  some  de- 
cided advantages.  Both  spiral 
(a)  and  cone  (b)  are  con- 
structed of  platinum.  The 
metallic  deposition,  it  will  be 
understood,  occurs  upon  the 
cone,  the  sides  of  which  are 
perforated,  so  that  a  uniform 

concentration  of  liquid  is  preserved  throughout  the  decom- 
position. When  liquid  electrolytes  contain  much  iron,  it  is 
essential  that  the  oxygen  liberated  within  the  cone  should 
be  equally  distributed  over  its  outer  surface.  This  is  made 
possible  through  openings.  The  shape  of  the  cone  also 
prevents  loss  from  the  bursting  of  the  bubbles  arising  from 
the  platinum  spiral  in  connection  with  the  anode. 

Krutwig  advises  adding  a  large  excess  of  ammonium  sul- 
phate to  the  silver  solution,  previously  made  alkaline  with 


io6 


ELECTRO-ANALYSIS. 


ammonium  hydroxide,  and  employs  a  current  of  N.D100  = 
0.02-0.05  ampere  and  2.5  volts.  In  this  way,  o.i  gram  of 
silver  may  be  precipitated  in  two  hours. 

The  writer's  experience  has  chiefly  been  with  solutions 
of  silver  containing  an  excess  of  a  pure  alkaline  cyanide. 
With  these  peroxide  separation  does  not  occur,  and  a  very 
weak  current  will  precipitate  0.15-0.20  gram  of  metal  in 
ten  hours  from  a  cold  solution.  If  the  liquid  be  heated  to 
65°  C.,  during  the  decomposition,  as  much  as  0.2-0.3  gram 
of  metal  may  be  precipitated  in  three  and  one-half  hours. 
The  current  density  for  this  precipitation  should  be  N.D100 
=  0.07  ampere.  Several  examples  from  a  student's  note- 
book will  show  how  well  the  method  works : — 


SILVER. 
GRAM. 

DILUTION. 
c.c. 

POTASSIUM 
CYANIDE. 
GRAMS. 

CURRENT. 
N.D100. 

VOLTS. 

TEMPERA- 
TURE. 

TIME 
HOURS. 

SILVER 
FOUND. 
GRAM. 

! 

0.2133 

125 

2 

0.03  A 

2-5 

65° 

4 

0.2132 

2 

0.2133 

125 

2 

0.03  A 

2-5 

60 

3 

0.2133 

3 

0.2133 

125 

4 

0.04  A 

2-5 

60 

3 

0.2131 

4 

0.2133 

125 

2 

O.O25A 

2.7 

60 

4 

0.2134 

5' 

0.2133 

1^5 

2 

O.O25A 

2.7 

60 

3 

0.2135 

6 

0.2133 

125 

2 

O.O25A 

2.7 

60 

4 

0.2125 

In  trials  i  and  2  the  metal  was  precipitated  upon  a  dish, 
while  in  3  and  4  a  plate  cathode,  and  in  5  and  6  a  cone  was 
used  to  receive  the  silver,  which  was  very  adherent,  and 
brilliant  in  lustre.  It  was  washed  with  water,  alcohol,  and 
ether. 

Chlorine,  bromine,  and  iodine  can  be  indirectly  estimated 
electrolytically  by  first  precipitating  them  as  silver  salts, 
then  dissolving  the  latter  in  potassium  cyanide,  and  exposing 
the  resulting  solution  to  the  action  of  a  current  from  three 
to  four  "  Crowfoot  "  cells. 

Luckow  reduced  silver  chloride  by  placing  it  in  a  platinum 


DETERMINATION    OF    METALS SILVER.  IO/ 

dish,  serving  as  the  negative  electrode,  covering  it  with 
dilute  sulphuric  or  acetic  acid,  and  allowing  the  positive 
electrode  to  project  into  the  solution.  Four  Meidinger  cells 
were  strong  enough  to  reduce  o.i  gram  of  silver  chloride 
in  ten  minutes.  The  deposit,  while  spongy,  was  adherent. 
It  was  washed  with  water  and  then  thoroughly  dried  to 
insure  the  absence  of  any  acid.  (See  the  reference  to 
Kinnicutt's  experiments;  also,  Prescott  and  Dunn,  Jr.  An. 
Ch.,  3,  373-) 

The  Rapid  Precipitation  of  Silver  With  the  Use  of  a 
Rotating  Anode. 

To  a  solution  of  silver  nitrate,  containing  0.4990  gram 
of  metal,  add  2  grams  of  potassium  cyanide,  heat  the  solu- 
tion (125  c.c.)  almost  to  boiling  and  electrolyze  with  a  cur- 
rent of  N.D100  =  2  to  2.8  amperes  and  5  volts.  The  metal 
will  be  precipitated  in  the  form  of  a  dense  white  deposit  in 
nine  to  ten  minutes.  Have  the  anode  perform  700  revo- 
lutions per  minute. 

The  rate  of  precipitation,  with  a  flat  spiral  anode,  from 
this  electrolyte  was  as  follows : 

In  i  minute 0.2046  gram 

In  2  minutes 0.3391   gram 

In  3  minutes 0.4858  gram 

In  4  minutes 0.5043  gram 

In  5  minutes 0.5225  gram 

In  7  minutes 0.5270  gram 

In  10  minutes 0.5301   gram 

By  using  the  dish  anode  described  on  p.  73  the  0.53  gram 
of  silver  present  was  precipitated  in  two  minutes,  all  but  a 
very  small  quantity  being  deposited  in  the  first  minute. 
Thus  with  5  volts  and  nine  to  ten  amperes  the  rate  of  precipi- 
tation was : 


108  ELECTRO-ANALYSIS. 

In  i  minute 0.5116  gram 

In  2  minutes 0.5304  gram 

In  3  minutes 0.5306  gram 

In  4  minutes 0.5306  gram 

One  fails  to  see  how  any  gravimetric  method  followed  in 
the  precipitation  of  silver  could  give  results  like  the  preced- 
ing. The  time  factor  is  almost  eliminated.  Every  part  of 
the  procedure  is  satisfactory. 

Gooch  and  Meday  also  obtained  very  excellent  determina- 
tions of  silver  by  depositing  it  upon  a  rotating  cathode 
(P-  47)- 

The  Rapid  Precipitation  of  Silver  With  the  Use  of  a 
Rotating  Anode  and  Mercury  Cathode. 

In  determining  silver  in  this  manner  have  it  in  the  form 
of  nitrate.  An  example  will  illustrate  the  best  conditions. 
To  5  c.c.  of  silver  nitrate  solution  (=0.2240  gram  of 
silver)  add  5  drops  of  nitric  acid  (30  drops  equaled  i  c.c.). 
Rotate  the  anode  at  a  speed  of  1200  revolutions  per  minute. 
At  the  end  of  five  minutes  the  precipitation  will  be  complete. 
Then  proceed  as  directed  in  all  determinations  made  in  this 
way. 

An  anodic  deposit  will  show  itself  in  the  first  minute 
or  two,  but  it  will  entirely  disappear  in  four  or  five  minutes. 
The  anode  should  have  a  high  speed  to  insure  agitation  of 
the  mercury  thereby  making  the  absorption  of  silver  more 
certain.  It  is  not  advantageous  to  have  a  greater  concen- 
tration than  0.3500  gram  of  silver  in  5  cubic  centimeters. 

The  rate  of  precipitation  in  this  electrolyte  was : 

In  i   minute 0.1874  gram  of  silver 

In  2  minutes 0.2178  gram  of  silver 

In  3   minutes 0.2207  gram  of  silver 

In  4  minutes 0.2240  gram  of  silver 


DETERMINATION    OF    METALS ZINC.  ICK) 


ZINC. 

LITERATURE. — Wright  son,  Z.  f.  a.  Ch.,  15,  303;  Parodi  and  Mas- 
cazzini,  Ber.,  10,  1098;  Z.  f.  a.  Ch.,  18,  587;  Riche,  Z.  f.  a.  Ch.,  17, 
216;  Beilstein  and  Jawein,  Ber.,  12,  446;  Z.  f.  a.  Ch.,  18,  588; 
Riche,  Z.  f.  a.  Ch.,  21,  119;  Reinhardt  and  I  hie,  Jr.  f.  pkt.  Ch.  [N. 
F.],  24,  193;  Classen  and  v.  Reiss,  Ber.,  14,  1622;  Gibbs,  Z.  f.  a. 
Ch.,  22,  558;  Luckow,  Z.  f.  a.  Ch.,  25,  113;  Brand,  Z.  f.  a.  Ch.,  28, 
581;  Warwick,'Z.  f.  anorg.  Ch.,  i,  258;  Vortmann,  Ber.,  24,  2753; 
Rudorff,  Z.  f.  ang.  Ch.,  Jahrg.  1892,  197;  Vortmann,  M.  f.  Ch.,  14, 
536;  v.  Malapert,  Z.  f.  a.  Ch.,  26,  56;  Her  rick,  Jr.  An.  Ch.,  2,  167; 
Jordis,  Z.  f.  Elektrochem.,  2,  138,  563,  655;  Millot,  B.  s.  Ch.  Paris. 
37,  339.'  v.  Foregger,  Dissertation,  Bern,  1896;  Rider er,  J.  Am.  Ch. 
S.,  21,  789;  Nicholson  and  A  very,  J.  Am.  Ch.  S.,  18,  659;  Pa  week, 
Berg-Hiitt.  Z.,  46,  S7o~573  ;  Pa  week,  Ch.  Z.  (1900),  24,  No.  80; 
Ho  Hard,  B.  s.  Ch.  Paris  (Series  3),  29,  262;  Ch.  N.  (1903),  87,  259; 
Amberg,  Ber.,  36,  2489  (1903);  Spitzer,  Z.  fur  Elektrochem.,  n, 
391;  C:urrie,  Ch.  N.,  91,  247;  Danneel  and  Nissenson,  Interna- 
tionaler  Congress  fur  angew.  Ch.  (1903),  4,  679;  Price  and  Judge, 
Ch.  N.,  94,  18;  Ingham,  J.  Am.  Ch.  S.,  26,  1269;  Jene,  Ch.  Z.,  29, 
801  ;  Exner,  J.  Am.  Ch.  S.,  25,  899;  Langness,  J.  Am.  Ch.  S., 
24,  463;  Kollock  and  Smith,  Am.  Phil.  Soc.  Pr.,  xliv,  137  (1905); 
Fischer  and  Bod'daert,  Z.  f.  Elektrochem.,  10,  946;  Foerster,  Z.  f. 
angw.  Ch.,  19,  1889  (1906);  Kollock  and  Smith,  Am.  Phil.  Soc.  Pr., 
45,  256. 

Much  has  been  written  upon  the  electrolytic  estimation 
of  zinc.  The  personal  experience  of  the  writer  inclines 
him  to  give  preference  to  the  method  suggested  by  Parodi 
and  Mascazzini.  They  recommended  that  the  metal  be 
present  in  solution  as  sulphate;  its  quantity  may  vary  from 
0.1-0.25  gram.  To  it  add  4  c.c.  of  a  solution  of  ammonium 
acetate,  20  c.c.  of  citric  acid,  and  dilute  to  200  c.c.  with 
water.  The  electrodes  are  then  introduced  into  the  liquid, 
their  distance  apart  being  not  more  than  a  few  millimeters. 
The  precipitation  can  be  made  in  a  beaker,  using  a  weighed 
platinum  cone  (Fig.  27)  as  the  cathode.  The  current  for 
this  purpose  should  be  0.5  ampere  and  5.9-6.3  volts.  At 


IIO  ELECTRO-ANALYSIS. 

5O°-6o°,  with  a  current  of  0.5  ampere,  the  pressure  will 
be  4.8-5.2  volts  and  the  deposit  of  metal  will  be  most  satis- 
factory. When  the  precipitation  of  metal  has  ended,  which 
may  be  ascertained  by  removing  a  small  quantity  of  the 
liquid  with  a  capillary  tube  and  bringing  it  in  contact  with 
a  drop  of  a  solution  of  potassium  ferrocyanide,  remove  the 
bulk  of  the  liquid  with  a  siphon.  Wash  the  deposit  with 
water  and  alcohol.  There  is  no  danger  of  oxidation  during 
the  drying  process.  It  will  be  discovered  on  dissolving 
the  precipitated  zinc  that  the  platinum  is  covered  with  a 
black  powdery  layer,  insoluble  even  in  hot  hydrochloric  or 
hot  nitric  acid.  This  is  platinum  black  (Vortmann,  Rii- 
dorff).  It  "is  exceedingly  difficult  to  remove,  and  to  pre- 
vent its  occurrence  it  is  best  to  coat  the  platinum  dish  with 
a  thin  layer  of  copper  or  silver  before  precipitating  the 
zinc  (p.  113). 

Beilstein  and  Jawein  add  sodium  hydroxide  to  the  solu- 
tions of  zinc  nitrate  or  sulphate,  until  a  precipitate  is  pro- 
duced, dissolve  it  in  potassium  cyanide,  and  dilute  with 
water  to  150  c.c.  The  decomposition  is  carried  out  in  a 
rather  large  beaker,  the  cathode  being  either  the  platinum 
cone  already  described  (p.  105),  or  a  rather  large  platinum 
crucible  suspended  from  a  cork,  perforated  by  a  copper 
wire,  touching  the  inner  surface  of  the  crucible.  If  the 
decomposition  takes  place  at  the  ordinary  temperature,  use 
a  current  of  N.D100  =  o.5  ampere  and  5.8  volts.  The 
precipitation  will  be  complete  in  from  two  to  two  and  one- 
half  hours.  It  may  be  reduced  to  one  and  one-half  to 
one  and  three-quarter  hours  by  heating  the  electrolyte  to 
60°  and  applying  a  current  of  the  density  just  given  and 
5  volts.  Wash  the  deposit  as  instructed  above. 

Reinhardt  and  Ihle  have  objected  to  nearly  all  the 
methods  which  have  been  proposed  for  the  electrolytic 


DETERMINATION    OF    METALS ZINC.  I  I  I 

estimation  of  zinc.  They  say  of  the  Beilstein  and  Jawein 
method  .  .  .  that  the  results  are  fairly  good,  .  .  .  but  a 
strong  current  is  necessary,  otherwise  the  precipitation  of 
the  zinc  is  slow  and  incomplete,  .  .  .  the  positive  pole  di- 
minishes in  weight  very  appreciably,  .  .  .  finally,  work- 
ing with  potassium  cyanide  is  very  unpleasant.  The 
writer's  experience  has  proved  that  a  current  considerably 
less  than  that  which  Beilstein  and  Jawein  first  recommended 
will  throw  out  all  the  zinc  in  the  course  of  a  night,  and 
further  that  the  anode  is  not  appreciably  affected.  The 
method  suggested  by  Reinhardt  and  Ihle  is,  however,  very 
excellent  and  deserves  trial  by  all  interested  in  the  electro- 
lytic estimation  of  zinc.  Its  essential  features,  taken  from 
their  publication,  are  these:  Mix  the  solution  of  zinc  sul- 
phate or  chloride,  neutral  as  possible,  with  an  excess  of 
neutral  potassium  oxalate,  until  the  precipitate,  which  appears 
at  first,  redissolves.  Or,  observing  the  recommendation  of 
Classen,  add  4  grams  of  potassium  or  ammonium  oxalate 
to  the  solution,  acidulate  the  latter  with  tartaric  acid 
(3:50),  dilute  to  150  c.c.  with  water,  heat  to  60°,  and 
electrolyze  in  copper-coated  platinum  dishes  with  N.D100  = 
0.5-1.5  amperes  and  3.5-3.8  volts.  Two  hours  will  be 
sufficient  for  complete  precipitation. 

The  immediate  decomposition  of  the  zinc  oxalate  is  into 
zinc  and  carbon  dioxide  (two  molecules),  and  the  potas- 
sium oxalate  into  carbon  dioxide  (two  molecules)  and 
potassium;  the  latter  then  reacts  with  the  water,  so  that 
while  an  abundant  liberation  of  hydrogen  occurs  at  the 
cathode,  the  alkali  simultaneously  set  free  is  converted  into 
acid  potassium  carbonate  by  the  carbon  dioxide  at  the 
anode : 

ZnC2O4  +  K2C2O4  =  (Zn ;+  2KOH  +  H2)  +  4CO2. 
Cathode.  Anode. 

2KOH  +  2CO,  =  2 


I  I  2  ELECTRO-ANALYSIS. 

Therefore,  just  as  long  as  zinc  oxalate  is  being  decom- 
posed, considerable  evolution  of  gas  is  noticeable  at  the 
positive  electrode,  and  when  this  diminishes,  and  occa- 
sional bubbles  escape,  the  decomposition  is  complete,  and 
the  deposition  of  metal  may  be  considered  finished. 

Free  oxalic  acid,  or  any  other  acid,  is  not  injurious  if 
there  is  a  sufficient  quantity  of  potassium  oxalate  present. 
Nitric  acid,  however,  free  or  combined,  should  be  avoided; 
it  gives  rise  to  ammonium  salts,  which  prevent  the  zinc 
from  separating  in  a  dense  form.  The  acid  potassium  car- 
bonate produced  during  the  decomposition  offers  great 
resistance  to  the  current;  it  is,  therefore,  advisable  to  add 
potassium  sulphate  to  the  solution  to  increase  its  conduc- 
tivity. Reinhardt  and  Ihle  recommend  the  following  solu- 
tions for  use  in  decompositions  like  that  just  described:  166 
grams  of  potassium  oxalate  in  i  liter  of  water;  250  grams 
of  potassium  sulphate  in  i  liter  of  water,  and  a  solution  of 
oxalic  acid  saturated  at  15°  C. 

•  Experiments. —  (i)  40  c.c.  of  a  solution  of  zinc  sulphate 
(  =0.1812  gram  of  metallic  zinc),  to  which  were  added  50 
c.c.  of  potassium  oxalate  and  100  c.c.  of  potassium  sulphate, 
were  electrolyzed  with  a  current  of  N.D100  =  0.3  ampere 
and  3.9-4.2  volts,  at  the  ordinary  temperature.  After  three 
to  four  hours  the  current  was  interrupted.  The  precipitated 
zinc  weighed  0.1814  gram.  (2)  2.1867  grams  of  brass 
(containing  tin,  copper,  lead,  and  zinc)  were  dissolved  in 
nitric  acid  and  the  tin  determined  in  the  usual  gravimetric 
way.  Its  quantity  was  found  to  be  0.04  per  cent.  In  the 
filtrate,  containing  nitric  acid,  lead  and  copper  were  deter- 
mined simultaneously  by  electrolysis  (the  copper  separated 
upon  the  cathode  and  the  lead  as  dioxide  upon  the  anode)  :— 

ra__0.8s%   Pb  and  64.60%    Cu. 
*oun(1  \fc_- 0.85%   Pb  and  64.62%   Cu. 


DETERMINATION    OF    METALS ZINC.  113 

The  acid  liquid  was  siphoned  off  from  the  deposits,  evap- 
orated to  dryness  with  sulphuric  acid,  neutralized  with 
caustic  potash,  and  then  to  this  ( 100  c.c.  in  volume)  solu- 
tion were  added  50  c.c.  of  a  solution  of  potassium  oxalate 
and  100  c.c.  of  a  solution  of  potassium  sulphate.  The  zinc 
found  equaled  34.50  per  cent. 

When  using  this  method  employ  a  stout  platinum  wire, 
wound  to  a  spiral  at  the  one  end,  for  the  anode,  and  a  plati- 
num cone  for  the  cathode  (p.  105).  To  avoid  the  peculiar 
spots  which  electrolytic  zinc  shows  upon  a  platinum  sur- 
face, it  will  be  best  to  first  coat  the  negative  electrode  with 
copper  (5  grams).  In  dissolving  the  precipitated  zinc,  use 
rather  dilute  nitric  acid.  The  copper  layer  will  be  but 
slightly  attacked,  and  after  washing  and  drying  will  serve 
for  further  depositions.  Wash  the  zinc  deposit  with  water, 
alcohol,  and  ether;  dry  in  a  desiccator.  Oxidation  is  liable 
to  occur  if  an  air-bath  be  used  for  the  drying. 

Jordis  prefers  lactic  to  oxalic  acid  in  the  electrolysis  of 
zinc  salts.  To  the  solution  containing  0.2  gram  of  metallic 
zinc  he  added  5  grams  of  ammonium  lactate,  2  grams  of 
lactic  acid,  and  5  grams  of  ammonium  sulphate.  The  liquid 
was  diluted  to  230  c.c.  and  acted  upon  at  60°  with  a  current 
of  N.Dj 00  =  o.io-o.23  ampere  and  3.4-3.9  volts.  The 
electrolyte  was  usually  agitated  (p.  97).  The  anode  and 
cathode  were  1.5  cm.  apart.  The  time  for  complete  preci- 
pitation occupied  four  and  a  quarter  hours.  A  copper- 
plated  platinum  dish  was  used  as  cathode. 

Nicholson  and  Avery,  adopting  the  suggestion  of  War- 
wick, add  3  c.c.  of  formic  acid  to  the  zinc  salt  solution,  then 
nearly  neutralize  with  sodium  carbonate,  dilute  to  150  c.c., 
and  electrolyze  at  the  ordinary  temperature  with  a  current 
varying  from  0.5  to  I  ampere. 

Millot,  Kiliani,  and  v.  Foregger  use  sodium  zincate  as 
n 


114  ELECTRO-ANALYSIS. 

electrolyte,  giving  the  following  example :  To  the  solution 
of  i  gram  of  zinc  sulphate  add  2  to  4  grams  of  sodium 
hydroxide,  dilute  to  125  c.c.  with  water,  heat  to  50°,  and 
electrolyze  with  N.D100  =  0.7-1.5  amperes  and  3.9-4.5 
volts.  All  of  the  metal  will  be  deposited  in  two  hours.  The 
character  of  the  deposit  is  improved  with  the  increase  in 
the  quantity  of  sodium  hydroxide.  In  applying  this  method 
to  the  determination  of  zinc  in  its  ores,  Jene  proceeds  as  fol- 
lows :  Dissolve  0.5  gram  of  the  ore  in  aqua  regia,  evaporate 
to  dryness,  add  I  to  2  c.c.  of  dilute  sulphuric  acid  ( i  :  i ) 
which  expel  by  heat.  When  the  mass  is  cold,  add  water, 
boil,  filter  and  wash  the  residue  with  hot  water.  The  filtrate 
should  not  exceed  80  to  100  c.c.  in  volume.  It  is  ready 
for  electrolysis.  Add  to  it  4  to  7  grams  of  solid  sodium 
hydroxide,  allowing  the  latter  to  dissolve  completely.  Heat 
to  50°  C,  and  electrolyze  without  any  regard  to  the  hydrox- 
ides swimming  in  the  solution.  Use  a  copper-plated  plati- 
num dish  with  N.D  100  =  i  ampere  and  a  pressure  of  from 
3.8  to  4.2  volts.  The  deposition  will  be  finished  in  from 
ij  to  2  hours.  The  end  of  the  decomposition  is  ascertained 
by  suspending  a  perfectly  clean  strip  of  sheet  copper  over 
the  edge  of  the  dish  and  observing  whether,  after  fifteen 
minutes,  it  has  become  coated  with  any  zinc. 

Riche  employs  "  a  solution  of  the  acetate  with  an  excess 
of  ammonium  acetate,  obtained  by  supersaturation  with 
ammonia  and  acidifying  with  acetic  acid."  This  method 
affords  good  results,  as  may  be  seen  from  the  following 
determination :  0.4736  gram  of  zinc  sulphate  was  dissolved 
in  200  c.c.  of  water,  to  which  were  added  3  grams  of  sodium 
acetate  and  10  drops  of  ordinary  acetic  acid.  When  there 
is  an  insufficiency  of  acetic  acid,  the  zinc  deposit  becomes 
spongy.  Ammonium  acetate  may  be  substituted  for  the 
sodium  salt.  After  two  hours  0.1063  gram  of  metallic 


DETERMINATION    OF    METALS ZINC.  IIS 

zinc  was  obtained,  the  required  quantity  being  0.1072  gram. 
The  temperature  should  be  60°  and  the  current  N.D100  = 
0.5  ampere  and  4.8-5.2  volts. 

Moore  seems  to  have  obtained  exceedingly  satisfactory 
results  by  precipitating  a  solution  of  zinc  sulphate  with 
sodic  phosphate,  then  adding  an  excess  of  ammonium  car- 
bonate, and  after  dissolving  the  precipitate  in  potassium 
cyanide,  the  solution  was  electrolyzed  at  a  temperature  of 
80°.  (See  method  of  Beilstein  and  Jawein.)  The  metal 
was  deposited  upon  a  silver-plated  electrode.  An  excellent 
•procedure,  originating  with  Luckow  and  previously  noticed 
in  the  Historical  section,  consists  in  introducing  0.5  gram  of 
metallic  mercury  into  the  dish  in  which  it  is  intended  to  elec- 
trolyze  the  solution  of  the  zinc  salt.  It  is,  of  course,  under- 
stood that  the  platinum  dish  and  the  drop  of  mercury  are 
weighed  together.  A  zinc  amalgam  is  precipitated ;  it  dis- 
tributes itself  in  a  beautiful  adherent  layer  over  the  surface 
of  the  dish. 

Paweck  believes  that  in  the  amalgam  method  suggested 
by  Vortmann  much  inconvenience  is  experienced  in  weigh- 
ing out  the  mercuric  chloride  and  subsequently  re-calcu- 
lating it  into  metal ;  further,  that  by  frequent  use  the  surface 
of  the  platinum  cathode  changes  to  spongy  platinum,  thus 
giving  rise  to  considerable  loss.  To  avoid  these  disadvant- 
ages he  suggests  the  use  of  amalgamated  zinc  or  brass  elec- 
trodes in  gauze  form.  The  introduction  of  these  eliminates 
the  addition  of  a  mercury  salt,  while  the  gauze  form  favors 
the  deposition  and  prevents  the  collection  of  hydrogen  bub- 
bles on  the  under  side  of  the  cathode,  whereby  a  spongy 
zinc  deposit  is  likely  to  be  produced.  The  gauze  electrodes 
are  semi-cylindrical  in  shape,  6  cm.  in  diameter,  two  being 
attached  to  a  brass  rod  at  a  distance  of  12  mm.  After  they 
have  been  cleaned,  they  are  amalgamated  or  coated  with 


I  1 6  ELECTRO-ANALYSIS. 

mercury  by  electrolyzing  a  solution  containing  0.6  gram  of 
mercuric  chloride.  The  amalgam  is  washed  with  alcohol, 
ether,  dried  and  weighed.  The  electrolyte  contains  the 
zinc  salt,  Seignette  salt  and  alkali.  It  may  be  electrolyzed 
with  a  current  of  0.1-0.5  ampere  and  2.6-3.6  volts.  The 
deposit  should  be  dried  at  3O°-4O°.  (See  p.  65.) 

Vortmann  has  found  that  zinc  may  be  readily  precipitated 
from  its  solution  in  the  presence  of  an  excess  of  sodium 
hydroxide  and  sodium  tartrate.  The  deposit  is  gray  in 
color  and  adheres  well  to  the  dish.  The  current  density 
(N.D100)  may  vary  from  0.3-0.6  ampere.  To  determine 
when  the  precipitation  is  complete,  remove  a  few  drops  of 
the  liquid  and  warm  with  ammonium  sulphide. 

The  Rapid  Precipitation  of  Zinc  With  the  Use  of  the 
Rotating  Anode. 

In  an  alkaline  electrolyte  (NaOH)  proceed  as  follows: 
To  25  c.c.  of  solution  (  =  0.2490  gram  of  zinc)  add  8 
grams  of  solid  sodium  hydroxide,  dilute  to  125  c.c.  with 
water,  heat  almost  to  boiling  then  remove  the  flame  and 
electrolyze  with  N.D100  =  5  amperes  and  6  volts.  The 
anode  should  make  about  600  revolutions  per  minute.  The 
precipitation  will  be  complete  in  twenty  minutes.  The  de- 
posit will  be  adherent,  smooth,  hard  and  gray  in  color.  The 
amount  of  sodium  hydroxide  may  vary  within  quite  wide 
limits. 

In  all  precipitations  of  zinc  in  platinum  vessels  coat  the 
latter  with  silver.  If  this  is  clone  one  such  coating  will 
serve  through  a  number  of  precipitations.  After  the  dish 
and  its  deposit  have  been  weighed  fill  the  dish  to  the  brim 
with  sulphuric  acid  previously  diluted  with  about  fifty  times 
its  volume  of  water,  then  set  the  dish  aside  until  the  action 
ceases.  Next  pour  the  solution  into  a  beaker,  rinse  the  dish 


DETERMINATION    OF    METALS ZINC.  I  I/ 

with  water  and  heat  it  to  faint  redness  over  a  free  flame 
while  holding  it  in  a  nickel  forceps.  Cool  under  a  faucet, 
fill  a  second  time  with  dilute  acid,  rinse  after  a  few  minutes, 
heat  as  before  and  give  a  third  treatment  with  the  same 
acid.  Finally,  after  rinsing  with  clean  water,  wipe  dry 
externally,  ignite,  cool  in  a  desiccator  and  weigh.  The 
entire  time  in  cleaning  the  dish  need  not  exceed  six  minutes. 
One  coat  of  silver  sufficed  for  more  than  a  hundred  deter- 
minations of  zinc. 

The  rate  of  precipitation  of  zinc  from  the  preceding  elec- 
trolyte, using  a  current  of  5  amperes  and  8  volts,  was— 

In    i  minute 0.1028  gram 

In    2  minutes 0.1847  gram 

In    3  minutes 0.2921   gram 

In    4  minutes 0.3498  gram 

In    5  minutes 0.421 7  gram 

In    7  minutes -. 0.4691  gram 

In  i  o  minutes 0.4740  gram 

In  1 2  minutes 0.4780  gram 

In  1 5  minutes 0.4780  gram 

In  an  alkaline  acetate  electrolyte  the  deposition  is  also 
very  rapid.  An  example  will  show  this — 

A  solution  of  zinc  sulphate,  equivalent  to  0.5004  gram  of 
metal,  containing  3  grams  of  sodium  acetate  and  0.2  c.c.  of 
acetic  acid  (30  per  cent.),  was  diluted  with  water  to  125  c.c. 
and  electrolyzed  with  a  current  of  N.D100  =  4  amperes  and 
10  volts.  In  fifteen  minutes  0.5002  gram  of  zinc  was  pre- 
cipitated on  the  silver-plated  platinum  dish.  The  deposit 
was  light  blue  in  color  and  crystalline.  The  anode  per- 
formed 600  revolutions  per  minute. 

Ingham  determined  the  rate  of  precipitation  of  zinc  from 
this  electrolyte: 


1 1 8  ELECTRO- ANALYSIS. 

In    i  minute  . . .. 0.0933  gram 

In    2  minutes 0.1500  gram     . 

In    3  minutes 0.2326  gram 

In    4  minutes 0.2957  gram 

In    5  minutes 0.3773  gram 

In    7  minutes 0.4645  gram 

In  i  o  minutes 0.4736  gram 

In  1 5  mihutes 0.4766  gram 

In  20  minutes 0.4779  gram 

when  the  amount  of  metal  in  the  electrolyte  equaled  0.4780 
gram. 

The  formate  electrolyte  was  prepared  as  follows : 
To  the  salt  solution  (=  0.2490  gram  of  zinc)  were  added 
5  grams  of  sodium  carbonate  and  4.6  c.c.  of  formic  acid, 
sp.  gr.  1.22.  The  solution  was  diluted  with  water  to  125 
c.c.,  heated  to  boiling  and  acted  upon  with  a  current  of 
N.D100  =  5  amperes  and  8  volts.  In  twenty  minutes  the 
entire  amount  of  metal  was  precipitated.  The  deposit  was 
fine-grained  and  very  adherent. 

The  rate  of  precipitation  was  found  to  be : 

In    i  minute 0.0839  gram  of  metal 

In    2  minutes 0.1418  gram  of  metal 

In    3  minutes 0.1723  gram  of  metal 

In    5  minutes 0.2095  gram  of  metal 

In    7  minutes 0.2244  gram  of  metal 

In  10  minutes 0.2464  gram  of  metal 

In  12  minutes 0.2483  gram  of  metal 

In  1 5  minutes 0.2490  gram  of  metal 

In  20  minutes 0.2490  gram  of  metal 

In  an  ammomacal  electrolyte  it  is  possible  to  precipitate 
the  metal  very  satisfactorily  by  using  a  rotating  anode.  It 
is  well  established  that  with  stationary  electrodes  the  same 
electrolyte  is  impracticable.  To  use  it  proceed  in  the  fol- 
lowing manner : 

Add  to  the  zinc  salt  solution  5  c.c.  of  hydrochloric  acid 


DETERMINATION    OF    METALS ZINC.  I -1 9 

(sp.  gr.  1.21),  25  c.c.  of  ammonium  hydroxide  (sp.  gr. 
0.95)  and  one  gram  of  ammonium  chloride.  Let  the  total 
dilution  be  125  c.c.  Electrolyze  with  N.D100  =  5  amperes 
and  5  volts.  In  twenty  minutes  a  quarter  of  a  gram  of 
metal  will  be  fully  precipitated.  The  deposit  will  be  all 
that  one  can  wish.  There  is  no  likelihood  of  the  anode 
being  attacked  by  the  chlorine.  This  electrolyte  can  be 
used  in  estimating  the  zinc  content  of  zincblende.  Weigh 
off  0.5  gram  of  the  powdered  ore  into  a  No.  5  porcelain 
dish,  moisten  it  with  water,  add  nitric  acid  (sp.  gr.  1.41) 
sufficient  to  cover  it  and  digest  upon  an  iron  plate.  In 
about  twenty  minutes  after  action  has  ceased  raise  the  cover 
enough  to  let  the  fumes  escape  and  rapidly  evaporate  the 
liquid  to  dryness.  Cover  the  residue  with  pure  hydro- 
chloric acid  (sp.  gr.  1.21)  and  again  evaporate  to  dryness. 
Repeat  the  treatment  with  hydrochloric  acid,  taking  care 
to  avoid  overheating  and  volatilization  of  any  chloride. 
Finally,  moisten  the  dry  salts  with  strong  hydrochloric  acid 
and  take  up  with  hot  water.  This  operation  need  not  re- 
quire more  than  an  hour  and  ten  minutes.  Having  filtered 
out  the  gangue,  precipitate  the  iron  with  ammonium  hy- 
droxide, receiving  the  filtrate  from  it  in  the  customary  sil- 
vered and  weighed  platinum  dish,  the  precipitate  not  being 
washed  with  water,  but  after  the  substitution  of  a  porcelain 
vessel  for  the  platinum  the  iron  hydrate  should  be  dissolved 
from  off  the  moist  filter  in  warm  dilute  acid  and  reprecipi- 
tated  with  ammonium  hydroxide.  Two  precipitations  will 
be  necessary  to  free  the  iron  completely  from  zinc.  To  the 
solution  in  the  platinum  dish  add  0.5  gram  of  ammonium 
chloride,  preferably  in  the  dry  form,  and  electrolyze  the 
solution  (125  c.c.  in  volume)  with  a  current  of  5  amperes 
and  6  volts.  Twenty  minutes  are  sufficient  for  the  precipi- 
tation. The  deposit  will  be  crystalline,  adherent  but  not 
spongy. 


1 20 


ELECTRO-ANALYSIS. 


By  this  method  the  zinc  content  of  a  blende  may  be  made 
in  a  little  more  than  two  hours  from  the  time  of  weighing 
off  the  powdered  ore  to  the  weighing  of  its  zinc  content. 

If  the  iron  in  the  ore,  after  removal  of  the  gangue,  is 
precipitated  as  the  basic  acetate  or  formate,  the  filtrate  from 
it  can  be  used  for  the  electrolytic  determination  of  the  zinc, 
using  the  rotating  anode.  The  results  will  be  most  satis- 
factory. 

The  Rapid  Precipitation  of  Zinc  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

This  metal  is  especially  readily  determined  in  this  manner. 
Perhaps  no  better  evidence  of  this  can  be  given  than  may  be 
found  in  the  accompanying  table  where  varying  condition? 
are  presented  in  detail. 

ZINC. 


6 

z 

L 

(/}  5 

Q 
U  u 
<C  U 
Z 

U  « 

IN  C-C. 

H  in 
z  a 

o 
m  * 

[INUTES 

z 

Q  . 

R 

K 

O 

w  < 

w 

tf  w 

J 

p  S  5 

^; 

O  < 

z 

X 

«  « 

P-0 

U 

•->  H 

X 

iJ 

o 

a! 

o 

P  a  z 

F 

U 

Z 

o 

X 

N 

£* 

H 

N 

« 

I 

0.2025 

0 

15 

7 

750 

30 

0.2027 

+  0.0002 

2 

0.2025 

0 

15 

7 

750 

25 

0.2030 

-f  0.0005 

3 

O.2O25 

0 

15 

7 

750 

25 

0.2015 

—  O.OOIO 

4 

O.2O25 

o 

15 

7 

750 

25 

0.2020 

—  O.OOO5 

5 

0.2025 

o 

15 

7 

750 

25 

0.2025 



6 

o.  2025 

o 

IO 

7 

750 

25 

O.2O24 

—  O.OOOI 

7 

O.2O25 

.25 

IO 

7 

750 

30 

o.  2027 

+  O.OOO2 

8 

0.4040 

.25 

20 

•5 

6 

750 

45 

0.2054 

+  O.OOO4 

9 

0.2025 

•25 

10 

5 

750 

25 

o  2025 



10 

0.2025 

•25 

10 

5 

750 

25 

/>  r* 

0.2029 

+  0.0004 

1  1 

O'2O25 

•25 

*  5 

75° 

25 

o.  2025 

12 

0.2025 

.25 

15 

5 

750 

20 

0.2027 

+  O.OOO2 

13 

O.2O25 

•25 

15 

2 

6 

750 

15 

0.2030 

-j-  O.OOO5 

O.2O25 

•25 

15 

2 

6 

750 

15 

0.  2020 

—  O.OOO5 

15 

O.2O25 

•25 

2 

6 

750 

15 

0.2021 

—  O.OOO4 

16 

0.4050 

.25 

15 

5 

8 

1,400 

6 

0.4057 

+  O.OO07 

17 

0.4050 

•25 

15 

5 

8 

480 

6 

0.4045 

-  0.0005 

18 

0.4050 

•25 

15 

5-6 

7-5 

480 

8 

0.4042 

—  O.OOOS 

19 

0.4050 

•25 

10 

5 

7 

640 

5 

0.4050 

DETERMINATION    OF    METALS ZINC.  121 

The  rate  of  precipitation  is  interesting : 

With  a  current  of  one  ampere  and  five  volts  acting  upon 
15  c.c.  of  a  zinc  sulphate  solution,  containing  0.2025  gram 
of  metal,  there  was  precipitated : 

In    5  minutes o.i  196  gram 

In  10  minutes , 0.1774  gram 

In  15  minutes 0.1897  gram 

In  20  minutes 0.2002  gram 

In  25  minutes 0.2027  gram 

With  a  like  volume  of  solution,  to  which  had  been  added 
0.4  c.c.  of  concentrated  sulphuric  acid;  a  current  of  two 
amperes  and  seven  volts,  precipitated : 

In    5  minutes 0.1860  gram  of  zinc 

In  10  minutes 0.1998  gram  of  zinc 

In  1 5  minutes 0.2020  gram  of  zinc 

On  dissolving  double  the  quantity  of  zinc  in  15  c.c., 
adding  0.25  c.c.  of  concentrated  sulphuric  acid,  a  current  of 
1.5  amperes  and  10  volts,  and  an  anode  rotating  at  the  rate 
of  800  revolutions  per  minute,  precipitated : 

In  10  minutes 0.3701   gram 

In  1 5  minutes 0.3997  gram 

In  20  minutes 0.401 1   gram 

In  30  minutes 0.4058  gram 

The  same  mass  of  zinc  in  twenty  cubic  centimeters  was 
electrolyzed  with  a  current  of  2  amperes  and  6  volts,  other 
conditions  being  identical,  at  this  rate : 

In  10  minutes 0.3352  gram 

In  15  minutes 0.4010  gram 

In  20  minutes 0.4030  gram 

In  30  minutes 0.4050  gram 

An  anode  rotating  at  440  revolutions  per  minute  and 
again  at  1000  revolutions  made  no  apparent  difference  in 
12 


122  ELECTRO-ANALYSIS. 

the  rate  at  which  the  metal  was  deposited.  The  mercury 
should  not  be  allowed  to  accumulate  too  much  of  the  metal 
— when  it  does,  results  are  not  obtained  so  quickly.  Con- 
centration of  the  electrolyte  is  most  favorable  to  rapid  and 
satisfactory  depositions  of  the  zinc  metal. 

NICKEL  AND   COBALT. 

LITERATURE. — Gibbs,  Z.  f.  a.  Ch.,  3,  336;  Z.  f.  a.  Ch.,  n,  10;  22,  558; 
Merrick,  Am.  Ch.,  2,  136;  Wright  son,  Z.  f.  a.  Ch.,  15,  300,  303,  333; 
Schweder,  Z.  f.  a.  Ch.,  16,  344;  Cheney  and  Richards,  Am.  Jr.  Sc. 
and  Ar.  [3],  14,  178;  Ohl,  Z.  f.  a.  Ch.,  18,  523;  Luckow,  Z.  f.  a.  Ch., 
19,  16  ;  Bergmann  and  Fresenius,  Z.  f.  a.  Ch.,  19,  314;  Riche,  Z. 
f.  a.  Ch.,  21,  116,  119;  Classen  and  v.  Reiss,  Ber.,  14,  1622,  2771*; 
Schucht,  Z.  f.  a.  Ch.,  22,  493;  Kohn  and  Woodgate,  Jour.  Soc. 
Chem.  Industry,  8,  256;  Riidorff,  Z.  f.  ang.  Ch.,  Jahrg.  1892,  p.  6; 
Brand,  Z.  f.  a.  Ch.,  28,  588;  Le  Roy,  C.  r.,  112,  722;  Vortmann,  M. 
f.  Ch.,  14,  536;  v.  Foregger,  Dissertation,  1896,  Bern;  Campbell  and 
Andrews,  J.  Am.  Ch.  S.,  17,  125;  Oettel,  Z.  f.  Elektrochem.,  i,  192; 
Fresenius  and  Bergmann,  Z.  f.  a.  Ch.,  19,  320;  Foster,  Z.  f.  Elektro- 
chem., 6,  160;  W inkier,  Z.  f.  anorg.  Ch.,  8,  291;  Hollar d,  B.  s.  Ch. 
[Series  3],  29,  22;  Danneel  and  Nissenson,  Internationaler  Congress 
fur  angw.  Ch.,  (1903)  4,  679;  Per  kin  and  Preble,  Ch.  N.,  90,  307; 
Exner,  J.  Am.  Ch.  S.,  25,  899;  Smith,  J.  Am.  Ch.  S.,  26,  1595; 
Kollock  and  Smith,  Am.  Phil.  Soc.  Pr.,  44  (1905),  137;  Fischer 
and  Bod'daert,  Z.  f.  Elektrochem.,  10,  946;  Foerster,  Z.  f.  angw. 
Ch.,  19,  1889  (1906);  Kollock  and  Smith,  Am.  Phil.  Soc.  Pr.,  45, 
262;  Fischer,  Z.  f.  Elektrochem.,  13,  361. 

These  metals  are  precipitated  from  solutions  of  their 
double  cyanides,  double  oxalates,  and  sulphates  mixed  with 
alkaline  acetates,  tartrates,  and  citrates,  or  from  ammoni- 
acal  solutions.  The  latter  seem  best  adapted  for  nickel 
depositions,  the  presence  of  ammonium  sulphate  or  sodium 
phosphate  being  favorable  to  the  precipitation. 

Fresenius  and  Bergmann,  who  have  carried  out  a  series 
of  experiments  with  nickel  and  cobalt,  give  the  following 
as  satisfactory  conditions:  50  c.c.  nickel  solution  (=  0.1233 


DETERMINATION    OF    METALS NICKEL,    COBALT.        123 

gram  of  nickel),  100  c.c.  of  ammonia  (sp.  gr.  0.96),  10  c.c. 
of  ammonium  sulphate  (305  grams  of  the  salt  in  i  liter 
of  water),  100  c.c.  of  water;  separation  of  the  electrodes 
J— J  cm.;  time,  four  hours.  The  current  was  N.D]00  = 
0.5-0.7  ampere  and  2.8-3.3  v°lts  at  tne  ordinary  tem- 
perature. The  nickel  found  weighed  0.1233  gram.  Ap- 
paratus suitable  for  the  decomposition  just  described  is 

FIG.  28. 


represented  in  Fig.  28.  The  metal  is  deposited  upon  the 
weighed  platinum  cone  in  the  beaker,  C.  The  vessel  is 
covered  with  a  glass  lid  having  suitable  apertures  for  the 
positive  and  negative  electrodes.  As  soon  as  the  blue- 
colored  liquid  becomes  colorless,  an  indication  that  the  metal 
is  completely  precipitated,  remove  a  few  drops  and  test  with 
a  solution  of  potassium  sulphocarbonate.  If  the  latter 
causes  only  a  faint  rose-red  coloration  the  deposition  of 
metal  may  be  considered  complete.  If  the  electrolysis  is 
unnecessarily  prolonged,  metallic  sulphide  may  be  produced 


1 24  ELECTRO-ANALYSIS. 

(Lehrbuch  der  analyt.  Chemie,  Miller  and  Kiliani).  It 
is  not  advisable  to  interrupt  the  current  or  to  remove  the 
cone  from  the  electrolyzed  liquid  until  the  latter  has  been 
replaced  by  water.  This  is  effected  by  the  vessels  to  the 
left  of  the  figure:  A  is  an  aspirator,  filled  with  water;  B 
is  air-tight  and  empty ;  x  is  a  doubly  bent  tube  extending  to 
the  bottom  of  C.  Open  p  and  the  liquid  in  C  is  gradually 
transferred  to  B.  Add  fresh  water  in  C.  Ammonium 
chloride  should  not  be  present  in  the  solution  undergoing 
electrolysis. 

Vortmann  adds  tartaric  or  citric  acid  and  an  excess  of 
sodium  carbonate  to  the  solution  of  the  nickel  salt,  then 
electrolyzes  with  a  current  density  of  N.D100  =  0.3-0.4 
ampere.  The  deposit  may  contain  traces  of  carbon. 

The  statements  upon  nickel  also  apply  to  cobalt.  An 
experiment,  taken  from  the  article  of  Fresenius  and  Berg- 
mann,  is  here  given  as  a  guide  in  determining  cobalt:  50 
c.c.  of  cobalt  sulphate  (=  0.1280  gram  of  cobalt),  100  c.c.  of 
ammonia,  10  c.c.  of  ammonium  sulphate,  100  c.c.  of  water; 
current  N.D100  =  0.5-0.7  ampere  and  2.8-3.3  vo^ts  at  tne 
ordinary  temperature;  separation  of  electrodes,  J-J  cm. 
Time,  five  hours.  The  deposited  cobalt  weighed  0.1286 
gram. 

Use  potassium  sulphocarbonate  to  test  when  the  metal 
is  fully  reduced;  it  gives  a  wine-yellow  coloration  with 
even  the  most  dilute  solutions  of  cobalt  salts. 

When  too  little  ammonia  is  present  in  the  electrolyte  the 
results  are  bad;  too  much  of  this  reagent  retards  the  deposi- 
tion of  the  cobalt. 

v.  Foregger  adds  15  to  20  grams  of  ammonium  car- 
bonate to  the  solution  of  i  gram  of  nickel  sulphate,  dilutes 
with  water  to  150  c.c.,  heats  to  60°,  and  electrolyzes  with 
N.D100=  1-1.5  amperes  and  3.5-4  volts.  Two  hours  will 
be  required  for  the  precipitation. 


DETERMINATION    OF    METALS NICKEL,    COBALT.        125 

Oettel  observed  that  nickel  could  be,  contrary  to  gen- 
eral statements,  as  well  precipitated  from  an  ammoniacal 
chloride  as  from  an  ammoniacal  sulphate  solution.  With 
a  current  of  N.D-,00  =  0.45  ampere  in  the  presence  of  40 
c.c.  of  free  ammonia  '(sp.  gr.  0.92),  10  grams  of  ammonium 
chloride  and  nickel  chloride  equivalent  to  1.0456  grams  of 
metal,  total  dilution  200  c.c.,  he  succeeded  in  throwing 
out  1.0462  grams  of  metal  in  six  and  one-quarter  hours. 
Nitric  acid  should  not  be  present.  More  difficulty  was 
experienced  with  cobalt.  The  most  favorable  results  were 
obtained  with  a  current  of  N.D100  =  0.4-0.5  ampere. 
The  quantity  of  ammonium  chloride  should  be  at  least 
four  times  that  of  the  cobalt  and  the  solution  should  con- 
tain one-fifth  of  its  volume  of  free  ammonia  (sp.  gr.  0.92). 
When  precipitating  these  metals  from  the  solutions  of  their 
double  oxalates,  the  conditions  should  be:  4  to  5  grams  of 
ammonium  oxalate,  120  c.c.  total  dilution,  temperature  60 °- 
70°,  with  N.D100  =  i  ampere  and  4  volts. 

The  writer  has  electrolyzed  cobalt  compounds  contain- 
ing an  excess  of  an  alkaline  acetate  (see  Zinc)  with  per- 
fectly satisfactory  results,  and  would  recommend  such  solu- 
tions for  this  particular  metal. 

In  this  laboratory  the  following  conditions  are  observed 
in  precipitating  nickel  from  a  cyanide  solution:  Add  o.i 
gram  more  of  alkaline  cyanide  than  is  necessary  for  the 
precipitation  and  re-solution,  2  grams  of  ammonium  car- 
bonate, dilute  to  150  c.c.,  heat  to  60°,  and  electrolyze  with 
N.D100=i.5  amperes  and  6-6.5  volts.  The  nickel  will 
be  fully  precipitated  in  three  and  one-half  hours.  Cobalt 
may  be  precipitated  under  similar  conditions. 

Sodium  pyrophosphate  precipitates  a  greenish-white  pyro- 
phosphate  from  nickel  solutions,  an  excess  of  the  reagent 
dissolves  the  precipitate,  while  the  liquid  becomes  yellow- 


1 26  ELECTRO-ANALYSIS. 

green  in  color.  The  latter  is  changed  to  green  by  am- 
monium carbonate,  and  to  blue  by  ammonium  hydroxide. 
When  electrolyzing  a  nickel  solution  add  to  it  20  c.c.  of  a 
sodium  pyrophosphate  solution,  25  c.c.  of  ammonia  (0.91 
sp.  gr.),  and  150  c.c.  of  water.  A  current  of  0.5  to  0.8 
ampere  will  be  sufficient  to  throw  out  the  nickel  in  nine 
hours.  This  method  will  serve  equally  well  for  the  estima- 
tion of  cobalt. 

In  determining  nickel,  Campbell  and  Andrews  dissolve 
nickel  hydrate  in  30  c.c.  of  a  10  per  cent,  solution  of 
sodium  phosphate,  add  30  c.c.  of  ammonia  to  the  same, 
dilute  to  125  c.c.  and  electrolyse  with  N.D100  =  o.i4  am- 
pere, the  electrodes  being  separated  5  mm.  The  precipita- 
tion is  complete  in  twelve  hours. 

The  Rapid  Precipitation  of  Nickel  With  the  Use  of  a 
Rotating  Anode. 

The  results  obtained  by  Exner  in  the  precipitation  of 
metals  with  the  aid  of  a  rotating  anode  have  led  to  a  most 
careful  investigation  of  the  best  conditions  for  each  metal. 
This  study,  with  nickel,  has  developed  most  interesting 
data  in  the  hands  of  West,  J.  Am.  Ch.  S.,  26,  1596.  The 
details  are  given  under  several  electrolytes.  The  condi- 
tions there  described,  if  adhered  to,  will  lead  to  the  most 
satisfactory  .results.  The  dilution  of  the  various  electro- 
lytes ranged  from  100  to  125  c.c.,  representing  a  cathode 
surface  of  100  sq.  cm.,  while  the  anode  performed  500  to 
600  revolutions  per  minute.  From  solutions  containing  an 
excess  of  ammonia  the  nickel  deposits  were  crystalline  and 
gray  in  color,  while  in  acid  solutions  the  metal  was  brilliant 
and  very  metallic  in  appearance — closely  resembling  the 
platinum.  Sometimes  peroxide  appeared  on  the  anode. 


DETERMINATION    OF    METALS NICKEL,    COBALT.       I2/ 

It  was  made  to  disappear,  in  ammoniacal  solutions,  by  add- 
ing more  ammonium  hydroxide  to  the  electrolyte,  and  if  it 
occurred  in  acid  solutions  by  lowering  the  current  toward 
the  end  of  the  decomposition,  and  after  a  few  minutes  again 
increasing  it,  or  by  introducing  into  the  acid  liquid  a  few 
drops  of  a  mixture  consisting  of  5  c.c.  of  glycerol,  45  c.c. 
of  alcohol  and  50  c.c.  of  water. 

In  an  ammoniacal  acetate  electrolyte  the  working  condi- 
tions should  be : 

For  0.4444  gram  of  nickel,  25  c.c.  of  ammonium  hydrox- 
ide (sp.  gr.  0.94),  10  c.c.  of  acetic  acid  and  125  c.c.  dilu- 
tion, a  current  of  N.D100  =  5  amperes  and  4.6  volts.  In 
twenty  minutes  the  metal  will  be  completely  precipitated. 
In  the  presence  of  sodium  acetate  and  free  acetic  acid  the 
precipitation  is  slower.  Thirty  minutes  were  necessary  for 
the  precipitation  of  the  quantity  of  metal  mentioned  in  the 
preceding  paragraph. 

In  an  electrolyte  of  ammonium  hydrate  and  ammonium 
sulphate,  which  is  the  time-honored  solution  for  the  deposi- 
tion of  nickel,  conditions  like  these  will  answer: 

Electrolyze  the  salt  solution  (containing  i.oioo  gram  of 
metal),  1.2  gram  of  ammonium  sulphate  and  30  c.c.  of 
ammonium  hydroxide  (sp.  gr.  0.94)  with  a  current  of  5.2 
amperes  and  6.5  volts.  The  precipitation  will  be  complete 
in  twenty-five  minutes. 

The  rate  of  precipitation,  using  a  solution  containing 
0.5050  gram  of  metal,  with  a  current  of  N.D100  =  4  am- 
peres and  5.5  volts  was: 

In  i  minute 0.0571  gram 

In  2  minutes o.i  164  gram 

In  3  minutes o. 1 549  gram 

In  4  minutes 0.2000  gram 

In  5  minutes 0.2510  gram 


128  ELECTRO-ANALYSIS. 

In    7^2  minutes 0.3580  gram 

In  10  minutes 0.4450  gram 

In  15  minutes 0.5007  gram 

In  20  minutes 0.5050  gram 

A  formate  electrolyte  answers  admirably  for  the  precip- 
itation of  nickel. 

To  a  solution  containing  0.4444  gram  of  metal,  add  20 
c.c.  of  ammonium  hydroxide  (0.094  sp.  gr.)  and  10  c.c.  of 
formic  acid,  then  electrolyze  with  a  current  of  N.D100  =  5 
amperes  and  4  volts.  All  of  the  metal  will  be  precipitated 
in  fifteen  minutes. 

Or,  the  metal  may  be  completely  precipitated  with  sodium 
carbonate  and  the  precipitate  be  dissolved  in  an  excess  of 
formic  acid.  For  example,  to  a  solution  of  nickel  sulphate 
(0.4444  gram  of  nickel)  add  five  grams  of  sodium  carbon- 
ate and  22  c.c.  of  formic  acid  (25  per  cent.),  then  elec- 
trolyze with  a  current  of  N.D100  =  5  amperes  and  4  volts. 
In  30  minutes  the  metal  will  be  completely  precipitated. 

The  rate  of  precipitation  in  this  electrolyte  was,  with  a 
current  of  5  amperes  and  4  volts,  as  follows : 

In    5  minutes 0.2474  gram 

In    7^  minutes 0.3260  gram 

In  10  minutes 0.3688  gram 

In  1 5  minutes , 0.4323  gram 

In  20  minutes 0.4394  gram 

In  30  minutes 0.4448  gram 

Nickel  is  quite  easily  determined  in  an  electrolyte  of 
ammonium  lactate.  Dilution  and  speed  should  be  the  same 
as  in  the  preceding  electrolytes. 

Conduct  a  current  of  5  amperes  and  7.5  volts  through 
the  solution  (containing  0.4444  gram  of  nickel),  in  which 
are  present  25  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.94) 
and  2.5  c.c.  of  lactic  acid.  The  precipitation  will  be  com- 
plete in  twenty  minutes.  The  rate  of  precipitation  is : 


DETERMINATION    OF    METALS NICKEL,    COBALT.       I 


In    5  minutes 0.3 151  gram 

In    75/2  minutes .0.4056  gram 

In  10  minutes 0.4344  gram 

In  1 5  minutes 0.4443  gram 

In  20  minutes 0.4443  gram 

The  Rapid  Precipitation  of  Nickel  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

In  the  experiments  given  in  the  subjoined  table  a  solu- 
tion of  nickel  sulphate,  equivalent  to  0.4802  gram  of  metal 
in  ten  cubic  centimeters,  was  used. 

NICKEL. 


I 

tfi    t/5 

a 

u 

U 

u 

to 

0 
m  ft! 

a 

S 

a 
z 

i 

cu 

y  u 

z 

1  i 

H 

I  "  H 

z 

fa  < 

o 

X 

H 

^  « 

^  z 

w 

S 

«  S 

J 
O 

3§5 

? 

wO 

5 

H 

X  « 

s  S 

3 

^  ~ 

• 

E 

«j 

O  ^1 

s  J^ 

H 

U  Z 

o 

y  -1 

>J 

o 

W 

^ 

K 

fc 

C/3 

|r 

ai 

H 

W 

j 

0.4802 

•25 

18 

2 

7 

600 

18 

0.4802 

2 

0.4802 

•25 

12 

3-5 

7 

600 

16 

0.4799 

—0.0003 

3 

0.4802 

•25 

12 

2-4 

6.5 

600 

IO 

0.4806 

-(-0.0004 

4 

0.4802 

•25 

12 

6 

5 

500 

7 

0.4804 

-fO.0002 

5- 

0.4802 

•25 

12 

5 

6.5 

600 

IO 

0.4796 

—0.0006 

6 

o.  9604 

•25 

IO-3O 

4 

6 

1,100 

IO 

0.9597 

—o  0007 

7 

0.4802 

•25 

12 

3 

7-5 

I,IOO 

IO 

0.4806 

-f  0.0004 

8 

0.4802 

•25 

12 

3 

7 

I,IOO 

IO 

0.4796 

—  0.0006 

9 

o.  9604 

•25 

12 

3-5 

7 

I,IOO 

16 

o.  9604 



10 

0.4802 

•25 

12 

5 

7 

640 

12 

0.4809 

-f  0.0007 

ii 

0.4802 

•25 

12 

5 

6 

880 

8 

0.4806 

4-  0.0004 

12 

0.4802 

•25 

7 

6 

5 

1,200 

9 

0.4801 

—  O.OOOI 

13 

0.4802 

•25 

7 

6 

6 

I,2OO 

7 

0.4801 

—  O.OOOI 

The  rate  of  precipitation,  when  using  a  current  of  2 
amperes  and  7  volts,  was  found  to  be : 

In    2y2  minutes 0.2017  gram  of  metal 

In    7^2  minutes 0.4095  gram  of  metal 

In  10  minutes 0.4651   gram  of  metal 

In  \2y2  minutes 0.4774  gram  of  metal 

In  1 5  minutes 0.4802  gram  of  metal 


1 3O  ELECTRO-ANALYSIS. 

A  nickel  solution  became  colorless  in  four  minutes  when 
exposed  to  a  current  of  6  amperes  and  5  volts.  Not  a 
trace  of  the  metal  was  present  in  the  solution  siphoned  off 
after  seven  minutes. 

Nickel  amalgam  is  very  bright  in  appearance.  A  gram 
of  the  metal  combined  with  the  usual  quantity  of  mercury 
(40  grams)  imparts  to  the  amalgam  the  consistency  of 
soft  dough. 

The  Rapid  Precipitation  of  Cobalt  With  the  Use  of  a 
Rotating  Anode. 

Various  electrolytes  have  been  studied  by  Miss  Kollock 
(J.  Am.  Ch.  S.,  26,  1606)  to  fix  more  definitely  the  con- 
ditions so  successfully  used  by  Exner.  The  results  con- 
clusively demonstrate  that  the  introduction  of  the  rotat- 
ing anode  has  given  the  electrolytic  method  of  estimating 
cobalt  a  very  superior  value.  The  details  in  procedure  are 
analogous  to  those  described  under  nickel. 

To  precipitate  it  from  a  sodium  formate  electrolyte,  add 
to  a  cobalt  sulphate  solution  (=0.3535  gram  of  metal) 
2.5  grams  of  pure  sodium  carbonate  and  4  c.c.  (94  per 
cent.)  formic  acid.  Heat  the  solution  to  boiling,  remove 
the  flame  and  electrolyze  with  a  current  of  N.D100  —  5 
amperes  and  6  volts.  The  precipitation  will  be  complete 
in  thirty  minutes.  The  deposit  of  cobalt  is  so  brilliant  that 
it  is  difficult  to  distinguish  it  from  the  platinum  on  which 
it  is  precipitated.  In  this  electrolyte  a  slight  anodic  deposit 
may  occur.  The  glycerol  mixture,  referred  to  under  nickel, 
causes  it  to  disappear  or  prevents  its  formation.  However, 
it  is  preferable  to  lower  the  current  to  one  ampere  for  a 
few  minutes  when  the  solution  has  nearly  lost  its  color. 
Just  as  soon  as  the  peroxide  has  disappeared  from  the 
anode  restore  the  current  to  its  original  strength.  Much 


DETERMINATION    OF    METALS NICKEL,    COBALT.        13! 

formic  acid  retards  the  precipitation.  If  the  liquid  becomes 
alkaline  the  deposition  is  very  rapid  and  the  metal  is  spongy, 
hence  add  the  acid  drop  by  drop  from  time  to  time. 

The  rate  of  precipitation  in  a  solution  containing  0.3152 
gram  of  cobalt  was : 

In    5   minutes 0.1470  gram  of  metal 

In    7 */2    minutes 0.2096  gram  of  metal 

In  10   minutes 0.2570  gram  of  metal 

In  1 5   minutes 0.3066  gram  of  metal 

In  20   minutes 0.3092  gram  of  metal 

In  25   minutes 0.3142  gram  of  metal 

In  30   minutes 0.3152  gram  of  metal 

By  applying  a  current  of  6.5  amperes  and  7  volts  to  a 
solution  containing  0.3152  gram  of  cobalt  in  the  presence  of 
20  c.c.  of  ammonium  hydroxide  and  3.5  c.c.  of  formic  acid 
(94  per  cent.)  all  of  the  metal  will  be  precipitated  in  twenty 
minutes.  If  the  solution  is  alkaline  the  metal  deposit  will 
be  very  compact  in  form  and  dull  in  appearance,  while 
if  the  liquid  is  acid  the  cobalt  will  separate  in  a  very  brilli- 
ant form,  but  more  slowly  than  from  an  ammoniacal  solu- 
tion. In  this  electrolyte — formate — there  is  little  tendency 
to  anodic  deposition. 

A  very  satisfactory  electrolyte  is  that  containing  am- 
monium acetate. 

Conduct  a  current  of  5  amperes  and  6  volts  through  a 
solution  of  cobalt  sulphate  (0.3310  gram  of  metal),  con- 
taining 25  c.c.  of  ammonium  hydroxide  and  10  c.c.  of  20 
per  cent,  acetic  acid.  The  metal  will  be  fully  deposited  in 
twenty-five  minutes.  It  will  be  brilliant  in  appearance  and 
there  will  be  no  sign  of  anodic  precipitation.  A  solution 
in  which  0.2980  gram  of  metal  was  present  gave  the  follow- 
ing rate  of  precipitation: 


I  3  2  ELECTRO-ANALYSIS. 

In    5  minutes 0.2235  gram  of  cobalt 

In  10  minutes 0.2778  gram  of  cobalt 

In  1 5  minutes 0.2950  gram  of  cobalt 

In  20  minutes . . . . 0.2980  gram  of  cobalt 

In  25  minutes 0.2980  gram  of  cobalt 

An  electrolyte  of  lactic  acid  or  a  lactate  will  also  answer 
admirably  in  the  estimation  of  this  metal.  Peroxide  pre- 
cipitation does  not  take  place.  The  cobalt  deposits  are  most 
adherent  and  exceedingly  brilliant  in  appearance.  A  large 
excess  of  lactic  acid  retards  the  precipitation. 

Add  to  the  solution  of  cobalt  sulphate  (=0.3152  gram 
of  metal),  2.2  grams  of  sodium  carbonate  and  5  c.c.  of 
concentrated  lactic  acid,  and  with  a  current  of  N.D100  =  5 
amperes  and  8  volts  the  precipitation  will  be  complete  in 
twenty-five  minutes. 

In  an  ammonium  lactate  solution  the  results  are,  if  any- 
thing, superior  to  those  in  the  preceding  electrolyte.  As 
a  rule  the  solution  becomes  colorless  in  twenty-five  minutes. 

To  a  solution  of  the  sulphate  (=  0.3310  gram  of  metal), 
add  30  c.c.  of  ammonium  hydroxide  and  7  c.c.  of  lactic 
acid  and  electrolyze  with  N.D100  =  6  amperes  and  5  volts. 
Twenty-five  minutes  will  suffice  for  complete  precipitation. 

The  rate  of  precipitation  was  found  to  be : 

• 

In    5   minutes 0.2215  gram  of  metal 

In  10   minutes 0.3060  gram  of  metal 

In  15   minutes 0.3230  gram  of  metal 

In  20   minutes 0.3290  gram  of  metal 

In  25    minutes 0.3310  gram  of  metal 

In  30   minutes 0.3310  gram  of  metal 

An  electrolyte  of  ammonium  succinate  can  be  employed. 
Some  carbon  is  apt  to  be  precipitated  with  the  cobalt. 
Sodium  succinate  should  not  be  used. 


DETERMINATION  OF  METALS NICKEL,  COBALT.   133 


The  Rapid  Precipitation  of  Cobalt  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

Cobalt  does  not  seem  to  enter  the  mercury  with  the  same 
rapidity  as  the  nickel  under  like  conditions.  The  appended 
table  presents  a  list  of  experiments.  By  duplicating  any 
one  of  them  satisfactory  results  may  be  expected.  Cobalt 
sulphate  was  the  salt  used : 

COBALT. 


h 
Z 
M 

!«! 

u 

o 

8 

Q 

t/i 

x 

ft.' 

c/)  ,/j 

Is 

(j 
z 

is 

g 

IS- 

D 
Z 

1  " 

PC 

O 

X 

K 

K  H 

M 

S  w 

tj 

Hyp 

0Q 

K 

z 

w 

hO 

5§ 

S 

§s 

0 

J       Q    j* 

Z 

S^ 

M 

<  Z 

a.  a 

J    • 

5x 

>  z  ^ 

M 

ffl   ^ 

O 

m  " 

Q 

D   K 

o 

U  *^ 

S 

,°   " 

• 

Cfl3" 

^ 

^ 

H 

^ 

M 

I 

0.3525 

•35 

15 

5 

7 

I25O 

15 

0.3522 

—0.0003 

2 

0.3525 

•25 

15 

3 

5 

980 

18 

0.3524 

O.OOOI 

3 

0.3525 

•25 

15 

4 

6 

600 

14 

0.3523 

—  O.OOO2 

4 

0.3525 

•25 

IO 

4 

6 

860 

16 

0.3530 

4-0.0005 

5 

0.3525 

•  5 

IO 

4 

6 

IOOO 

15 

0-353° 

4-  0.0005 

6 

O.3525 

.0 

IO 

4 

6 

1240 

16 

0.3528 

4-0.0003 

7 

0.3525 

•25 

IO 

3 

6 

I2OO 

IO 

0.3521 

—  0.0004 

8 

0.3525 

•5 

IO 

6 

6 

I2OO 

IO 

0.3.530 

4-0.0005 

9 

0.3525 

•25 

IO 

5 

8 

800 

IO 

0.3522 

—0.0003 

10 

0.3525 

•25 

IO 

3 

8 

I4OO 

12 

0.3523 

—  0.0002 

ii 

0.3525 

•5 

IO 

6 

5 

800 

II 

0.3530 

4-0.0005 

12 

O.7O5O 

•  -5 

15 

6 

1200 

30 

0.7052 

-f  0.0002 

13 

o.  1762 

•35 

10 

4 

8 

560 

7 

0.1762 

J 

A  solution  of  cobalt  chloride  may  also  be  used  (p.  89). 
Thus,  introduce  into  the  mercury  cup  5  c.c.  of  a  cobalt 
chloride  solution  (=  0.1250  gram  of  metal),  cover  the  same 
with  10  c.c.  of  pure  toluene  and  electrolyze  with  a  current 
of  from  2  to  4  amperes  and  5  volts.  In  five  minutes  the 
liquid  will  be  colorless,  and  the  metal  will  be  completely 
precipitated  in  7  minutes. 


1 34  ELECTRO-ANALYSIS. 

MANGANESE. 

LITERATURE. — Z.  f.  a.  Ch.,  n,  14;  Riche,  Ann.  de  Chim.  et  de  Phys. 
[5th  ser.],  13,  508;  Luckow,  Z.  f.  a.  Ch.,  19,  17;  Schucht,  Z.  f.  a.  Ch., 
22,  493;  Classen  and  v.  Reiss,  Ber.,  14,  1622;  Moore,  Ch.  N., 
53,  209;  Smith  and  Frankel,  Jr.  An.  Ch.,  3,  385;  Ch.  N.,  60,  262; 
Brand,  Z.  f.  a.  Ch.,  28,  581;  Riidorff,  Z.  f.  ang.  Ch.,  Jahrg.  15,  p.  6; 
Classen,  Ber.,  27,  2060;  En  gels,  Z.  f.  Elektrochem.,  2,  413;  3,  286; 
Groeger,  Z.  f.  ang.  Ch.  (1895),  253;  Kaeppel,  Z.  f.  anorg.  Ch.,  16, 
268;  Currie,  Ch.  N.,  91,  247;  Koster,  Z.  f.  Elektroch.  10,  553; 
Scholl,  J.  Am.  Chem.  S.,  25,  1045,  Koster,  Z.  f.  Elektrochem.,  10 
(1904),  553- 

The  electric  current  causes  this  metal,  when  in  solution 
as  chloride,  nitrate,  or  sulphate,  to  separate  as  the  dioxide 
upon  the  anode  (see  Lead).  In  a  solution  of  nitric  acid, 
the  hydrogen  set  free  reduces  the  acid  to  oxides  of  nitro- 
gen and,  finally,  to  ammonia.  Under  such  conditions  com- 
plications may  arise,  particularly  if  other  metals  are  present 
in  the  solution.  For  this  reason  a  solution  of  the  sulphate, 
slightly  acidulated  with  two  to  six  drops  of  sulphuric  acid, 
is  preferable  for  electrolytic  purposes.  Neumann  prefers 
the  mineral  acid  solutions  for  these  depositions,  and  gives 
the  following  as  illustrative  examples : 

(a)  To  the  solution  containing  0.3  gram  of  manganese 
nitrate,  add  2  c.c.  of  concentrated  nitric  acid,  dilute  to  150 
c.c.  with  water,  and  electrolyze  with  N.D100  =  0.3  ampere 
and  3-3.5  volts  for  two  hours.     It  is  advisable  to  acid  the 
acid  during  the  course  of  the  electrolysis.     When  its  quan- 
tity exceeds  3  per  cent,  the  permanganic  acid  reaction  shows 
itself. 

(b)  Add  0.5  c.c.  of  concentrated  sulphuric  acid  to  the 
solution  of  0.3  gram  of  manganese  sulphate,  dilute  to  150 
c.c.,  heat  to  60° -70°,  and  act  upon  the  solution  for  four 
hours  with  a  current  of  0.4-0.6  ampere  and  4  volts. 


DETERMINATION    OF    METALS MANGANESE.  135 

As  soon  as  the  manganese  has  been  fully  precipitated  as 
dioxide,  the  current  is  interrupted,  the  deposit  washed  with 
water,  and  should  any  of  the  dioxide  become  detached,  it 
must  be  caught  upon  a  small  filter,  then  dried,  ignited,  and 
weighed,  together  with  the  adherent  dioxide,  which  is 
changed  to  protosesquioxide  (Mn3O4)  before  weighing. 
Groeger  has  demonstrated  by  iodometric  tests,  that  the  com- 
position of  the  precipitate  only  approximates  the  formula — 
MnO2.H2O — usually  assigned  it.  Further,  it  is  useless  to 
try  to  obtain  a  definite  compound  by  drying.  The  product 
is  so  extremely  hygroscopic  that  ignition  alone  to  the  pro- 
tosesquioxide will  give  definite  and  concordant  results. 

In  the  presence  of  large  quantities  of  iron,  this  precipita- 
tion is  unsatisfactory;  therefore,  first  remove  the  iron  with 
barium  carbonate.  Tartaric,  oxalic,  and  lactic  acids  retard 
the  formation  of  manganese  dioxide.  The  same  is  true  of 
phosphoric  acid.  Potassium  sulphocyanide  also  prevents  its 
formation,  and  if  added  to  solutions  in  which  dioxide  is 
already  precipitated,  it  causes  the  same  to  redissolve. 

Classen  maintains  that  strong  mineral  acids,  such  as  nitric 
and  sulphuric,  retard  the  complete  deposition  of  the  manga- 
nese. He  regards  acetic  acid  as  the  most  suitable  of  all 
the  organic  acids  for  use  in  this  precipitation.  The  condi- 
tions given  are :  25  c.c.  of  acetic  acid  of  specific  gravity 
1.069;  75  c-c-  °f  water;  temperature,  5o°-68° ;  N.D100  = 
0.3-0.35  ampere;  ¥  =  4.3-4.9;  time,  3  hours;  roughened 
dish. 

Engels  dissolves  the  manganese  salt  in  50  c.c.  of  water, 
adds  10  grams  of  ammonium  acetate  and  ij— 2  grams  of 
chrome  alum,  then  dilutes  with  water  to  150  c.c.,  heats  to 
80°,  and  applies  a  current  of  N.D100  =  0.6-0.9  ampere  and 
3-4  volts.  The  deposit  is  washed  with  water  and  alcohol, 
then  dried  and  ignited.  The  deposition  was  made  in  rough- 


1 36  ELECTRO-ANALYSIS. 

ened  dishes  of  platinum.  Alcohol  (5-10  c.c.)  may  be  sub- 
stituted for  the  chrome  alum,  but  more  time  will  then  be 
required  for  the  precipitation. 

Kaeppel  has  given  the  precipitation  of  manganese 
thoughtful  consideration,  fie  confirms  the  experience  of 
Engels,  and  adds  that  acetone  is  a  very  desirable  addition. 
This  method  of  procedure  consists  in  heating  the  electro- 
lyte to  55°,  adding  1.5  to  10  grams  of  acetone,  and  electro- 
lyzing  with  a  current  of  N.D100  =  0.7-1.2  amperes  and 
4-4.25  volts  for  a  period  of  from  two  to  five  hours.  The 
acetone  is  converted  into  acetic  acid,  and  it  is  the  transi- 
tional formation  of  the  latter  that  the  author  regards  as 
more  beneficial  in  the  deposition  than  if  it  be  added  directly 
to  the  electrolyte. 

In  this  laboratory  a  formate  electrolyte  has  been  used 
with  good  results.  Thus,  to  a  manganous  sulphate  solu- 
tion (=  o.i  100  gram  of  metal)  were  added  five  cubic 
centimeters  of  formic  acid  (specific  gravity  1.06),  10  c.c.  of 
a  sodium  formate  solution  (  =  i  gram  of  the  salt),  the 
whole  was  diluted  to  130  c.c.  with  water  and  electrolyzed 
with  a  current  of  N.D100=i.4  ampere  and  a  pressure 
ranging  from  12  volts  at  the  beginning  to  8.6  volts  at 
the  end.  The  precipitation  was  finished  at  the  expiration 
of  one  and  a  half  hours.  The  deposit  of  dioxide  was  very 
adherent. 

Later  it  was  observed  that  the  deposition  could  be  satis- 
factorily made  in  the  presence  of  free  formic  acid  alone. 
The  pressure  was  at  the  start  high,  because  of  the  low  con- 
ductivity of  the  formic  acid.  It  fell  in  the  course  of  an 
hour.  An  example  from  many  will  give  the  conditions. 
To  a  solution  containing  0.2068  gram  of  manganese  there 
were  added:  5  c.c.  of  formic  acid  (sp.  gr.  1.09)  and  it  was 
electrolyzed  at  room  temperature  with  N.D100  =  0.8  to  i 


DETERMINATION    OF    METALS MANGANESE. 


37 


ampere  and  6.8  volts.  The  time  required  was  five  hours. 
The  manganese  weighed  0.2069  gram.  The  deposit  from  a 
formate  electrolyte  is  very  adherent.  Formic  acid  is  supe- 
rior to  acetic  acid  as  an  electrolyte.  For  the  separation  of 
manganese  from  iron  and  from  zinc  see  pp.  262,  266. 

FIG.  29. 


The  apparatus  devised  by  Herpin  (Fig.  29)  can  be  well 
applied  in  the  decomposition  of  manganese  salts.  It  con- 
sists of  a  platinum  dish,  A,  resting  upon  a  tripod,  B,  in  con- 
nection with  the  cathode  of  a  battery.  The  upper  portion 
of  the  dish  is  so  constructed  that  it  will  support  an  inverted 
glass  funnel,  D.  Any  loss  from  the  bursting  of  bubbles  is 
13 


I38  ELECTRO-ANALYSIS. 

prevented  by  this  means.  The  anode  is  a  platinum  spiral 
C.  In  estimating  manganese  it  must  not  be  forgotten  to 
connect  the  dish  with  the  anode  of  the  battery  employed  for 
the  decomposition. 

The  Rapid  Precipitation  of  Manganese  With  the  Use 
of  a  Rotating  Electrode. 

The  experiments  made  in  this  direction,  in  this  laboratory, 
were  not  successful.  Koster  has  proposed  the  following : 

To  the  electrolyte,  about  130  cubic  centimeters  in  volume, 
containing  the  manganese  salt  (not  the  chloride)  add  5  to 
10  grams  of  ammonium  acetate,  2  to  3  grams  of  chrome 
alum  and  several  cubic  centimeters  of  alcohol.  Heat  the 
solution  to  75°  C.,  remove  the  flame  and  electrolyze  with 
N.D100  =  4  to  4.5  amperes  and  a  pressure  of  7  volts. 
Another  suggestion  from  the  same  chemist  consists  in  add- 
ing to  the  solution  of  the  manganese  salt  10  grams  of 
ammonium  acetate  and  about  10  cubic  centimeters  of  96 
per  cent,  alcohol.  The  current  density  and  pressure  to  be 
used  are  dependent  upon  the  quantity  of  manganese  present. 
For  example,  in  the  case  of  0.2  gram  of  manganese  or  less, 
use  a  current  of  N.D100  =  4  to  4.5  amperes  and  7  to  8 
volts;  when  there  is  a  larger  quantity  use  but  2  amperes 
and  4  to  5  volts.  The  author  declares  that  in  the  presence 
of  more  than  0.3  gram  of  manganese  neither  suggestion,  as 
given  above,  can  be  relied  upon,  because  oxide  will  detach 
itself  even  from  a  sand-blasted  electrode.  The  time  re- 
quired for  precipitation  varies  from  20  to  25  minutes. 

IRON. 

LITERATURE. — Wright  son,  Z.  f.  a.  Ch.,  15,  305;  Parodi  and  Mas- 
cazzini,  G.  ch.  ital.,  8,  178;  also  Z.  f.  a.  Ch.,  18,  588;  Luckow,  Z.  f.  a. 
Ch.,  19,  18;  Classen  and  v.  Reiss,  Ber.,  14,  1622;  Classen,  Z.  f. 


DETERMINATION    OF    METALS IRON.  139 

Elektrochem.,  i,  288;  Moore,  Ch.  N.,  53,  209;  Smith,  Am.  Ch.  Jr., 
!O,  330;  Brand,  Z.  f.  a.  Ch.,  28,  581  ;  Drown  and  McKenna,  Jr.  An. 
Ch.,  5,  627  ;  Smith  and  M  u  h  r ,  Jr.  An.  Ch.,  5,  488 ;  Rtidorff,  Z.  f.  ang. 
Ch.,  15,  Jahrg.,  p.  198;  Vortmann,  M.  f.  Ch.,  14,  536;  Heidenreich, 
Ber.,  29,  1585;  A  very  and  Dales,  Ber.,  32,  64,  2233;  Verwer  and 
Groll,  Ber.,  32,  37,  806;  Goecke,  Dissertation,  Bonn,  1900;  Kollock, 
J.  Am.  Ch.  S.,  21,  928;  Exner,  J.  Am.  Ch.  S.,  25,  903;  Kollock 
and  Smith,  Am.  Phil.  Soc.  Pr.,  44,  149;  ibid.,  45,  261. 

The  suggestion  of  Parodi  and  Mascazzini  relative  to  the 
precipitation  of  iron  (p.  28)  has  since  been  elaborated  by 
Classen,  and  by  him  applied  to  many  other  metals.  Fol- 
lowing the  recommendation  of  this  chemist,  about  six  to 
seven  grams  of  ammonium  oxalate  are  dissolved  in  as  little 
water  as  possible,  and  the  iron  salt  solution  gradually  added 
to  it  with  constant  stirring.  The  liquid  is  then  diluted  with 
water  to  150-175  c.c.,  and  electrolyzed  at  the  ordinary  tem- 
perature with  a  current  of  N.D100=  1.5  amperes  and  2-4.5 
volts,  or  at  the  temperature  of  4O°-65°  with  0.5-1.0  ampere 
and  2-3.5  volts.  If  ferric  hydroxide  should  separate  during 
the  electrolytic  decomposition,  it  can  be  redissolved  by  add- 
ing oxalic  acid  drop  by  drop.  Test  the  clear  liquid,  acidu- 
lated with  hydrochloric  acid,  with  potassium  sulphocyanide. 
The  deposited  iron  has  a  steel-gray  color;  it  should  be 
washed  with  water,  alcohol,  and  ether.  Avoid  the  presence 
of  chlorides  and  nitrates.  By  carefully  complying  with  the 
conditions  recommended  by  Classen  good  results  are  sure 
to  follow.  To  show  that  persons  with  but  little  experience 
do  succeed  with  the  preceding  method  the  two  following 
determinations,  made  by  a  student,  are  given :  A  quantity 
of  ferric  ammonium  sulphate  (=0.0814  gram  of  iron) 
was  dissolved  in  200  c.c.  of  water,  and  to  this  were  added  8 
grams  of  ammonium  oxalate.  The  solution  was  heated  to 
80°,  and  in  two  hours,  with  a  current  of  1.5  amperes, 
0.0814  gram  of  iron  was  obtained.  In  a  second  experi- 


ELECTRO-ANALYSIS. 

ment  the  quantity  of  iron  was  doubled  (  =0.1628  gram  of 
iron),  while  the  ammonium  oxalate  was  n  grams,  tem- 
perature 66°,  and  the  current  i  ampere.  The  precipitated 
iron  weighed  0.1619  gram  instead  of  0.1628. 

The  writer  found  the  following  procedure  admirably 
suited  for  iron  determinations :  10  c.c.  iron  solution  (  = 
0.1277  gram  of  metal),  10  c.c.  sodium  citrate  (1.8  grams) 
with  3  c.c.  of  citric  acid  (0.059  gram)>  tnen  diluted  with 
water  to  250  c.c.,  and  electrolyzed  with  a  current  of  N.D100 
=  0.8  ampere  and  7-8  volts  at  50°  for  four  and  one-half 
hours.  The  iron  deposit  weighed  0.1280  gram.  It  con- 
tained 0.94  per  cent,  of  carbon.  The  deposit  was  washed 
as  already  directed.  In  several  determinations  aluminium 
and  titanium  were  present  with  the  iron,  but  the  latter  was 
precipitated  free  from  the  other  two.  For  this  reason  the 
writer  regards  the  method  as  useful.  E.  F.  Kern,  working 
in  this  laboratory  with  the  view  of  arriving  at  some  knowl- 
edge in  regard  to  the  carbon  deposition,  after  long  and 
painstaking  experimentation,  recommends  the  following 
conditions  as  favorable  for  the  getting  of  iron  deposits  free 
from  the  carbon  impurity :  Add  i  gram  of  sodium  citrate 
and  o.i  gram  of  citric  acid  to  the  solution  of  iron  sulphate 
(o.i  gram  of  metal),  dilute  to  150  c.c,,  heat  to  60°,  and 
electrolyze  with  N.D100  =  0.8-1.3  amperes  and  9  volts. 
Just  as  soon  as  the  iron  is  precipitated,  siphon  off  the  liquid 
and  wash  without  interruption  of  the  current.  The  opinion 
exists  that  prolonged  action  of  the  current  after  the  metal  is 
all  deposited  tends  to  increase  the  carbon  content  of  the  iron. 

From  ammoniacal  tartrate  solutions  iron  is  also  precipi- 
tated, but  carries  carbon  with  it.  It  would  therefore  not  be 
advisable  to  use  this  electrolyte  except  in  cases  where  sepa- 
rations were  desired,  which  were  possible  only  in  solutions 
of  this  character. 


DETERMINATION    OF    METALS IRON.  H! 

A  third  method,  originated  by  Moore,  advises  that  glacial 
phosphoric  acid  (15  per  cent,  acid)  be  added  to  the  distinctly 
acid  solution  of  ferric  chloride  or  sulphate,  until  the  yellow 
color  fully  disappears,  then  a  large  excess  of  ammonium 
carbonate  is  added  and  a  gentle  heat,  is  applied  until  the 
liquid  becomes  clear.  On  electrolyzing  the  hot  (70°)  solu- 
tion with  a  current  of  2  amperes,  the  iron  is  rapidly  and 
completely  deposited  at  the  rate  of  0.75  gram  per  hour. 
Avery  and  Dales,  on  the  other  hand,  claim  that  with  a  cur- 
rent of  N.D100  =  2  amperes  and  5  volts  they  were  not  able 
to  precipitate  more  than  0.2  gram  of  iron  in  five  hours. 
The  end  of  the  decomposition  is  recognized  by  testing  a 
portion  of  the  solution  with  ammonium  sulphide.  Wash 
the  deposit  as  already  directed. 

Recently,  quite  a  little  discussion  has  been  had  upon  the 
deposition  of  iron  and  its  enclosures.  Avery  and  Dales 
question  whether  the  metal  is  fully  precipitated  from  any 
one  of  the  electrolytes  described  in  the  preceding  para- 
graphs; furthermore,  they  affirm  that  even  from  an  oxalate 
solution  the  iron  carries  down  carbon  with  it;  that  oxalic 
acid  is  converted  in  part,  at  least,  into  glycollic  acid,  and  that 
iron  salts  in  the  presence  of  the  latter  acid  yield  upon  elec- 
trolysis a  metal  strongly  contaminated  with  hydrocarbons. 
As  to  Moore's  method,  they  assert  that  phosphorus  is  always 
present  in  the  deposit  of  iron.  Goecke  concurs  with  these 
chemists  in  their  views  on  the  cathodic  contaminations. 
Verwer  and  Groll  think  that  iron,  from  an  oxalate  solution, 
is  absolutely  free  from  carbon,  while  Classen  attributes  the 
trifling  amounts  of  carbon,  which  have  been  observed,  to 
carelessness  and  inexperience  in  the  execution  of  the  pre- 
scribed directions. 

Consult  Blum  and  Smith,  Am.  Phil.  Soc.  Pr.,  46,  59, 
on  the  cathodic  precipitation  of  carbon. 


142  ELECTRO- ANALYSIS. 

Drown,  pursuing  a  suggestion  made  by  Wolcott  Gibbs 
in  1883  relative  to  the  precipitation  of  metals  in  the  form 
of  amalgams,  has  applied  it  to  the  determination  of  iron. 
The  trial  tests  were  made  with  a  solution  of  ferrous  ammo- 
nium sulphate,  slightly  acidulated  with  sulphuric  acid,  to 
which  a  large  excess  of  mercury  was  added  (not  less  than 
fifty  times  the  weight  of  the  iron  to  be  precipitated).  A 
large  platinum  anode  was  used,  while  the  mercury  cathode 
was  brought  into  the  circuit  by  means  of  a  platinum  wire 
enclosed  and  fused  into  one  end  of  a  glass  tube  which  passed 
through  the  liquid.  The  current  employed  for  the  precipi- 
tation equaled  about  2  amperes  per  minute.  The  author 
remarks  that  if  these  conditions  be  observed,  as  much  as  10 
grams  of  iron  can  be  precipitated  in  from  ten  to  fifteen 
hours. 

The  decomposition  was  carried  out  in  beakers.  Care 
should  be  exercised  in  drying,  so  that  no  mercury  is  vola- 
tilized. 

The  Rapid  Precipitation  of  Iron  With  the  Use  of  a 
Rotating  Anode. 

The  only  electrolyte  from  which  this  metal  was  deposited, 
while  using  a  high  current  and  high  pressure,  was  that  of 
ammonium  iron  oxalate.  The  anode  performed  800  revo- 
lutions per  minute  and  the  other  conditions  may  be  learned 
from  two  actual  trials. 

i.  To  a  solution  of  ferric  ammonium  sulphate  (0.2461 
gram  of  iron)  were  added  7.5  grams  of  ammonium  oxalate 
and  one  cubic  centimeter  of  a  saturated  solution  of  oxalic 
acid.  This  was  then  electrolyzed  after  heating  to  boiling 
with  a  current  of  N.D100  =  7  amperes  and  7.5  volts.  In 
twenty-five  minutes  0.2461  gram  of  iron  was  precipitated. 
The  deposit  of  metal  was  very  dense  and  so  light  in  color 


DETERMINATION    OF    METALS IRON.  143 

that  it  resembled  the  polished  platinum  dish  on  which  it  was 
precipitated. 

2.  In  this  trial  all  the  conditions  were  like  those  in  i, 
excepting  the  quantity  of  iron  equaled  0.4922  gram.  In 
thirty-five  minutes  this  exact  amount  of  metal  was  obtained. 

No  attempt  thus  far  has  been  made  to  determine  the  rate 
of  precipitation  of  iron  from  this  electrolyte. 

The  Rapid  Precipitation  of  Iron  With  the  Use  of  the 
Rotating  Anode  and  Mercury  Cathode. 

In  carrying  out  this  precipitation  an  example  will  give  the 
most  satisfactory  information : 

Five  cubic  centimeters  contained  0.2075  gram  of  iron. 
Three  drops  (40  drops  =  i  cubic  centimeter)  of  concen- 
trated sulphuric  acid  were  added  to  it,  when  it  was  electro- 
lyzed  with  a  current  of  3  to  4  amperes  and  7  volts.  The 
anode  made  from  500  to  900  revolutions  per  minute.  The 
iron  was  completely  deposited  in  seven  minutes.  The 
water  was  then  siphoned  off  and  the  amalgam  washed  as  in 
all  previous  cases  with  alcohol  and  water. 

The  rate  of  precipitation,  under  the  conditions  just  men- 
tioned, was : 

In  2  minutes 0.1760  gram  of  iron  was  deposited 

In  4  minutes 0.2000  gram  of  iron  was  deposited 

In  6  minutes 0.2050  gram  of  iron  was  deposited 

In  8  minutes 0.2075  gram  of  iron  was  deposited 

The  following  table  exhibits  conditions  which  can  be  re- 
lied upon : 


144 


ELECTRO-ANALYSIS. 


B- 

Q      '  •' 

u 

s* 

s 

Is 

g 

Q 

M 

»  < 

«  2  'I 

2 

Z     [I] 

K    BJ 

B 

O       H 

S  S 

O 

M 

o  5 

2; 

£  „ 

s  z  § 

^  s 

o 

3  o  5 

g 

fe  K 

zO 

M 

o2 

ft<   M  ^ 

j 

u< 

^  ^ 

M 

o 

0 

M 

s      0 

O 

M 

M 

M 

M 
M 

w    J* 

* 

H 

W 

I 

0.2075 

7 

5 

4    -5 

8    -7 

520 

14 

0.2072 

—  o.  0003 

2 

0.2075 

4 

5    -4 

6.5-5 

680 

H 

0.2078 

-(-0.0003 

3 

0.2075 

5 

5-10 

3-2-4 

6-5 

680 

15 

0.2077 

—  0.0003 

4 

0.2075 

3 

5 

2    -2.5 

7-6 

680 

15 

0.2073 

—  0.0002 

5 

0.2075 

3 

5 

4 

6-5 

680 

IO 

0.2080 

40.0005 

6 

0.2075 

3 

5 

3    -4-5 

7-6 

92O 

7 

0.2078 

40.0003 

7 

0.2075 

3 

5 

2    -3 

6 

740 

9 

0.2076 

4O.OOOI 

8 

0.2075 

3 

5 

2    -4 

6-5-5-5 

700 

9 

0.2076 

4  0.0001 

When  the  metal  exists  as  chloride  this  salt  may  be  electro- 
lyzed  with  ease,  taking  the  precaution  to  add  to  the  electro- 
lyte a  layer  of  pure  toluene  (p.  89).  For  example,  to  5 
cubic  centimeters  of  a  pure  ferric  chloride  solution 
(=0.1030  gram  of  iron),  were  added  10  cubic  centimeters 
of  toluene  and  the  liquid  electrolyzed  with  a  current  of 
two  to  four  amperes  and  nine  volts.  In  twelve  minutes 
the  total  quantity  of  metal  had  entered  the  mercury. 


CHROMIUM. 

LITERATURE. — Myers,    J.    Am.    Chem.    S.,    26,    1128;    Kollock    and 
Smith,  Am.  Phil.  Soc.  Pr.,  44,  146. 

This  metal  has  never,  until  recently,  been  determined  in 
the  electrolytic  way.  Upon  experimenting  with  a  solution 
of  its  sulphate  it  was  found  that  chromium  would  enter  or 
attach  itself  to  a  mercury  cathode,  accordingly  a  solution 
of  this  salt  was  electrolyzed  in  the  mercury  cup  (p.  58), 
using  stationary  electrodes.  Ten  cubic  centimeters  of  the 
salt  solution  contained  0.1080  gram  of  chromium.  The 
working  conditions  are  shown  in  the  following  table: 


DETERMINATION    OF    METALS CHROMIUM. 


</i 

2 

I 

J 

M        O 

CONDITIONS 

SO 

II 

J 

«J2 

1 

S  Z 

o  ~ 

|E 

h  w 

S  H  " 

H 

i 

•i 

H 

M 
M 

12' 

X  Q 

H 

J 

j 

rj  W 

O 

^  C/2  W 

s 

0. 

O 

E 

0 

H 

g 

55 

s>^S 

H 

1 

> 

jj 

1 

04 

fa 

^     ^ 

I 

O.IO8O 

0.1079 

2 

2 

3 

o-3 

7 

0-55 

5-5 

2 

O.IO8O 

0.1080 

I 

3 

14 

0.3 

7 

°-55 

5-5 

3 

0.2160 

0.2157 

I 

4 

14 

0.4 

7-5 

0.7 

6 

4 

0.  2  1  60 

0.2160 

I 

4 

0.4 

7-5 

0.7 

6 

5 

0.3240 

0.3235 

I 

8 

30 

0.7 

7 

2. 

6.5 

6 

0.3240 

0.3222* 

1 

6 

30 

0.65 

7 

2-5 

8 

The  initial  voltage  and  amperage  are  given  to  the  left  in 
the  table.  The  acid  liberated,  during  the  course  of  the  elec- 
trolysis, causes  the  potential  to  fall  and  the  current  to  rise 
to  the  final  voltage  and  amperage  exhibited  on  the  right. 
Chromium  amalgam  is  not  very  stable.  Water  rapidly 
decomposes  it  with  the  separation  of  metallic  chromium  as 
a  fine  black  powder  on  the  surface  of  the  mercury.  The 
amalgam  must,  therefore,  be  washed  as  rapidly  as  possible. 
A  given  amount  of  mercury  should  not  be  used  for  more 
than  one  decomposition.  The  appearance  of  an  oxide  of 
chromium  in  the  electrolyte  indicates  an  insufficient  amount 
of  acid. 

The  Rapid  Precipitation  of  Chromium  With  the  Use  of 
the  Rotating  Anode  and  Mercury  Cathode. 

To  10  cubic  centimeters  of  chromium  sulphate  (=  0.1180 
gram  of  metal),  add  three  drops  of  concentrated  sulphuric 
acid  (40  drops  =  I  cubic  centimeter),  and  electrolyze  with 
a  current  of  from  4  to  5  amperes  and  6  volts,  the  speed  of 
the  anode  being  400  revolutions  per  minute.  Six  minutes 
will  more  than  suffice  for  the  complete  precipitation  of  the 

*  Some  chromium  floated  off  in  wash  water. 
14 


146 


ELECTRO-ANALYSIS. 


metal.     Siphon  off  the  acid  liquid,  and  wash  the  amalgam 
as  quickly  as  possible  •  with  anhydrous  alcohol  and  ether. 

The  following  table  shows  conditions  which  may  be  relied 
upon  to  yield  results  that  will  be  satisfactory  in  every  way : 


g  u! 

u 

h 

o 

g 

a 

</> 

H 
Z 

U 

IN 

1; 

u 

z 

z'  & 

in  K 

*S« 

| 

^  < 

•'  o 

K 
H 

III 

1° 

w 
p 

I! 

KO 

3  |  5 

Z 

If  ' 

w 

u1^  .5 

"s3 

tJ 

o 

u.<- 

$«< 

u 
S 

K  ™ 

§  . 

7.^ 

M 

H 

u 

w  - 

J 

0.1180 

5 

10-15 

3-4 

7 

280 

15 

0.1186 

40.0006 

2 

0.1180 

3 

10-15 

2-4 

n   -9 

280 

15 

0^1187 

4-0.0007 

3 

o.  1  1  80 

3 

10-15 

9 

640 

20 

0.1185 

-J-O.OOO5 

4 

o.  1180 

3 

8-15 

'•5-3 

10  -8 

220 

15 

0.1186 

-j-  O.OOO6 

5 

o.  1180 

3 

10-15 

ii   -9 

520 

20 

0.1186 

4-O.OOO6 

6 

o.  1180 

3 

5-15 

1-2 

ii   -9 

640 

17 

0.1175 

—  O.OOO5 

7 

0.1180 

3 

5-15 

2-4 

9  -8 

480 

15 

o.  1180 



8 

0.2360 

3 

5-15 

2-5 

10 

520 

50 

0.2355 

—  O.OOO5 

9 

0.1180 

5 

5-15 

3 

7.5 

400 

15 

0.1179 

—  O.OOOI 

10 

0.1180 

3 

7-15 

4   -5 

8 

640 

6 

0.1175 

—  0.0005 

T   T 

at  T  8r* 

7T  C 

3                     A 

tc\       n 

f\AC\ 

IO 

oil  80 

1   1 

.  1  1  OU 

1J 

~"4 

ILJ    —  y 

\JQ\J 

12 

o.  1180 

7 

7-15 

3    -4 

io  -8 

200 

13 

0.1187 

4  0.0007 

13 

0.1180 

3 

5-15 

3-5 

8 

640 

II 

0.1177 

—  0.0003 

0.2360 

4 

5-15 

3 

12 

640 

35 

0.2359 

—  O.OOOI 

15 

o.  1180 

3 

5-15 

3    -4 

io  -8 

32O 

ii 

0.1179 

—  O.OOOI 

16 

0.1180 

3 

5-15 

3    -4 

IO 

540 

u 

o.  1182 

40.0002 

The  rate  of  precipitation,  deduced   from  these  figures, 
would  be : 

In    2  minutes 0.0480  gram  of  metal 

In    4  minutes 0.0850  gram  of  metal 

In    6  minutes o.idoo  gram  of  metal 

In    8  minutes. o.i  105  gram  of  metal 

In    9  minutes 0.1185  gram  of  metal 

In  io  minutes 0.1185  gram  of  metal 


URANIUM. 

LITERATURE.— L u c k o w ,  Z.  f.  a.  Ch.,  19,  18;  Smith,  Am.  Ch.  Jr.,  i, 
329;  Smith  and  Wallace,  J.  Am.  Ch.  S.,  20,  279;  Kollock  and 
Smith,J.  Am.  Ch.  S.,  23,  607  ;  K  e  r  n ,  J.  Am.  Ch.  S.,  23,  685  ;  Wherry 
and  Smith,  J.  Am.  Ch.  S.,  29,  806. 


DETERMINATION    OP    METALS URANIUM. 


For  electrolytic  purposes  use  the  acetate,  the  sulphate, 
or -the  nitrate.  Connect  the  dish  in  which  the  deposition 
is  made  with  the  negative  electrode  of  the  battery.  The 
uranium  separates  as  yellow  uranic  hydroxide  upon  the 
cathode;  by  the  continued  action  of  the  current  it  changes 
to  the  black  hydrated  protosesquioxide.  As  soon  as  the 
solution  becomes  colorless,  interrupt  the  current,  wash 
with  a  little  acetic  acid  and  boiling  water;  dry,  ignite,  and 
weigh  as  protosesquioxide.  If  any  of  the  hydrate  becomes 
detached,  collect  the  same  upon  a  small  filter,  and  ignite 
the  latter  together  with  the  dish  contents.  Conditions  lead- 
ing to  successful  results  are  contained  in  the  following 
examples : 

ELECTROLYSIS  OF  URANIUM  ACETATE. 


f  j 

l/l 

H" 

H 

U 

u 

o 

K    ' 

a-  . 

s 

§1 

U 

h 
u 

H 

I 

O 
ffi 

P  S 
o  < 

M 
0 

00 

U 

o 

1 

X 

X 

^ 

H) 
0 

< 

z 

w' 

g 

ocO 

z 

tt 

o 

«  ** 

0. 

^ 

• 

J—  .       M 

£) 

M    (J 

Q 

H 

^ 

K 

"*< 

H 

W    ,  , 

0.0986 

0.2 

I25 

ND      -029A 

16.25 

70 

5 

0.0988 

-f  0.0002 

0.0986 

0.2 

125 

N;D^  =  b.3  A 

12.2 

70 

5 

0.0989 

-f  0.0003 

0.1972 

0.2 

125 

N.D107--^o.3  A 

10-75 

70 

6 

o.  1970 

—  0.0002 

0.2298 

O.I 

N.D107^o.o9A 

4-25 

70 

6 

0.2297 

-0.000  1 

0.2298 

0.2 

125 

4.25 

70 

sX 

0.2299 

-f  0.0001 

ELECTROLYSIS  OF  URANYL  NITRATE  SOLUTIONS. 


U308 
PRESENT, 
IN  GRAMS. 

DILUTION 
c.c. 

TEMPERA 

TURK      °C. 

CURRENT. 

VOLTS. 

TIME. 
HOURS: 

U308 
FOUND  IN 
GRAMS. 

0.1222 
0.1222 

125 
125 

H 

N.D)07  =  o.o35A 
N.D107  =  o.o4  A 

4.6 

2.25 

5^ 

7^ 

0.1225 
O.I2I8 

Quantitative  results  were  also  obtained  by  the  electrol- 
ysis of  the  sulphate.     The  neutral  salt  solution  was  diluted 


148 


ELECTRO-ANALYSIS. 


to  125  c.c.  and  heated  to  75°  C,  when  a  current  of  from 
0.02  to  0.04  ampere  for  107  sq.  cm.  of  cathode  surface  and 
2.25  volts  was  conducted  through  the  liquid. 

ELECTROLYSIS  OF  URANYL  SULPHATE. 


h* 

u 

u 

JS 

. 

1 

•  03 

p 

§ 

£  !/> 

H 

H  *j 

u 

H 
<!  • 

8 

w 

O  <; 

0 

^O 

o 

CURRENT. 

O 

oo'J 

5 

°I« 

3 

1 

> 

u 

°«5 

M 

o 

& 

Q 

H 

H 

M 

0.1320 

125 

75 

N  F>     n  n?  A 

2 

6X 

0.1320 

IM.1JI07-  0.02  A 

0.1320 

125 

75 

N.D]07r=:0.02  A 

2 

5/4 

o.  1322 

+  0.0002 

o.i393 

125 

75 

N.DlW=o.04  A 

2.25 

5 

0.1395 

+  6.0002 

o.i393 

125 

70 

N.D,07=o.o38A 

2.25 

7 

0.1392 

—  0.000  1 

This  method  affords  an  excellent  separation  of  uranium 
from  the  alkali  and  alkaline  earth  metals  (p.  271). 


H 

Q 

z 

2 

Q 

6 

U 

U 

ISi 

u  w 

1 

5S 

H  5 

OH* 

|1 

fc 

^o 

h  u 

O  H  K 

si 

o 

7.  Z 

H 

co^ 

°«S 

u 

^ 

0^ 

HS 

£" 

I 

0.1527 

O.2 

2/2 

3 

H 

18 

ord. 

O.I5I3 

2 

0.1527 

0.2 

4/4^ 

3 

12 

15 

" 

O.I525 

3 

0.2613 

0.25 

5/^ 

7 

15 

8 

60° 

O.26II 

4 

0.2613 

0.25 

4K 

4 

12 

3 

50 

0.0344 

5 

0.2613 

0.25 

4)4 

4 

12 

15 

50 

0.0530 

6 

0.2613 

0.25 

4/4 

4 

12 

IO 

50 

0.1074 

7 

0.2613 

0.25 

4/4 

4 

12 

18 

50 

0.1935 

8 

0.2613 

0.25 

41A 

4 

12 

25 

50 

0.2467 

9 

0.2613 

0.25 

4/2 

4 

12 

30 

50 

0.26II 

a    . 
H  -y) 

<JP 

<  Z 

U  M 

IO 

0.2613 

I 

5 

15 

25 

o.  2600 

II 

0.2613 

2 

5 

13 

3° 

0.2613 

DETERMINATION    OF    METALS THALLIUM.  149 

The  Rapid  Precipitation  of  Uranium  With  the  Use 
of  a  Rotating  Anode  (performing  600  revolutions  per 
minute)  may  be  seen  in  the  results  on  the  preceding  page, 
obtained  when  using  a  uranyl  sulphate  solution. 

Either  of  the  two  electrolytes  mentioned  here  will  prove 
quite  satisfactory,  and  the  procedure  cannot  fail  to  com- 
mend itself  to  mineral  analysts. 

THALLIUM. 

LITERATURE. — Schucht,  Z.  f.  a.  Ch.,  22,  241,  490;  Neumann,  Ber., 
21,  356;  Heiberg,  Z.  f.  anorg.  Ch.,  35,  346. 

This  metal  separates  as  sesquioxide,  from  acid  solutions, 
upon  the  anode,  while  from  ammoniacal  liquids  it  is  de- 
posited partly  as  metal  and  partly  as  oxide.  From  oxa- 
late  solutions  and  from  its  double  cyanides  it  separates 
only  as  metal  when  the  current  is  feeble.  However,  diffi- 
culty is  experienced  in  drying  the  deposit  without  having 
it  oxidized.  In  this  respect  it  is  even  more  troublesome 
than  lead.  Neumann  utilizes  the  current  to  separate  the 
metal,  dissolves  the  latter  in  acid,  and  measures  the  liberated 
hydrogen;  from  its  volume  he  calculates  the  quantity  of 
thallium  originally  present.  For  suitable  apparatus  to  carry 
out  this  method  consult  the  literature  cited  above. 

The  recommendation  of  Heiberg  is  that  to  a  solution  of 
thallium  sulphate  (0.2  to  i.oooo  gram  of  salt)  in  100  c.c. 
of  water  there  be  added  2  to  6  c.c.  of  normal  sulphuric 
acid  and  5  to  10  c.c.  of  acetone.  Use  a  roughened  dish 
which  is  made  the  anode  during  the  decomposition.  Heat 
to  55°  C.,  and  electrolyze  with  a  current  ranging  from  0.02 
to  .05  ampere  and  pole  pressure  of  1.7  to  2.3  volts. 

The  precipitation  is  finished  when  |-  c.c.  of  the  electrolyte 
produces  no  opalescence  on  bringing  it  into  3  to  5  c.c.  of 


I    O  ELECTRO-ANALYSIS. 

a  five  per  cent,  solution  of  potassium  iodide.  Pour  out 
the  liquid  quickly  from  the  dish  and  wash  the  deposit  of 
oxide  several  times  with  water,  alcohol,  and  ether.  Dry 
for  twenty  minutes  at  i6o°-i65°  in  an  air  bath.  Cool  in 
a  desiccator.  The  time  for  precipitation  is  about  seven 
hours.  The  oxide  is  T12O3. 

Recently,  G.  W.  Morden,  working  in  this  laboratory, 
found  that  the  most  satisfactory  course  to  pursue  in  esti- 
mating thallium  electrolytically  consists  in  precipitating  it 
with  the  aid  of  the  rotating  anode  and  mercury  cathode. 
If  the  metal  is  precipitated  directly  into  the  mercury  the 
resulting  amalgam  will  on  washing  give  up  a  portion  of 
its  thallium  content  to  the  water.  This,  however,  may  be 
absolutely  prevented  by  precipitating  a  little  zinc  simul- 
taneously in  the  mercury.  Indeed,  as  small  a  quantity  as 
0.0007  §"ram  of  zinc  will  prevent  any  oxidation  of  as  much 
as  0.1305  gram  of  thallium.  To  the  solution  of  the  sul- 
phates contained  in  the  mercury  cup  add  a  few  drops  of 
sulphuric  acid  (specific  gravity  1.8)  and  electrolyze  with 
a  current  of  5  amperes  and  n  volts.  In  10  minutes  as 
much  as  0.2250  gram  of  thallium  may  be  precipitated  and 
the  amalgam  washed  and  dried  in  the  customary  way. 


INDIUM. 

LITERATURE. — T'hiel,    Z.    f.    anorg.    Chemie,    39,    119;    Dennis    and 
Geer,  Ber.,  37,  175;  J.  Am.  Ch.  S.,  26  (1904)^  438. 

Thiel  asserts  that  indium  may  be  determined  in  the  elec- 
trolytic way  with  great  accuracy.  He  recommends  that 
it  be  deposited  on  a  silver-plated  platinum  cathode. 

Dennis  and  Geer  found  that  this  metal  may  be  readily 
precipitated  from  solutions  of  its  chloride  or  nitrate  in  the 
presence  of  pyridine,  hydroxylamine  or  formic  acid.  The 


DETERMINATION    OF    METALS PLATINUM.  I$I 

depositions  from  oxalic  or  oxalate  solutions  were  not  very 
satisfactory.  The  metal  separated  from  an  acetate  elec- 
trolyte in  a  dark,  spongy  form,  while  from  solutions  con- 
taining pyridine  it  was  brilliant  white  in  color  and  very 
compact. 

In  making  a  determination  dissolve  the  yellow  oxide  in 
one-sixth  normal  sulphuric  acid,  avoiding  an  excess.  Add 
to  this  solution  25  cubic  centimeters  of  formic  acid  (spe- 
cific gravity  1.20)  and  5  cubic  centimeters  of  ammonia 
(specific  gravity  0.908),  then  dilute  to  200 cubic  centimeters, 
and  electrolyze  with  a  current  of  N.D100  =  9  to  12  amperes. 
The  quantity  of  metal  varied  from  0.2  to  1.5  gram.  It  was 
deposited  on  a  rotating  cathode — a  roughened  dish.  The 
cathode  will  not  be  attacked  so  long  as  the  electrolyte  con- 
tains formic  acid. 

PLATINUM. 

LITERATURE. — Luckow,  Z.  f.  a.  Chv  19,  13;  Classen,  Ber.,  17,  2467; 
Smith,  Am.  Ch.  Jr.,  13,  206;  Riidorff,  Z.  f.  ang.  Ch.,  1892,  696; 
Langness,  J.  Am.  Ch.  S.,  29,  466. 

The  solutions  of  platinum  salts,  slightly  acidulated  with 
sulphuric  acid,  and  acted  upon  by  a  feeble  current,  give 
up  the  metal  as  a  bright,  dense  deposit  upon  the  dish, 
frequently  so  light  as  to  be  scarcely  distinguished  from  the 
latter.  In  using  platinum  vessels  for  this  purpose,  first 
coat  them  with  a  rather  thick  layer  of  copper,  upon  which 
afterward  deposit  the  metal.  Wash  the  deposit  with  water 
and  alcohol. 

In  ordinary  gravimetric  analysis,  potassium  is*  frequently 
estimated  as  potassio-platinum  chloride,  K2PtCl6.  This 
operation  requires  time  and  care.  Rather  dissolve  the 
double  salt  in  water,  slightly  acidulate  the  solution  with 
sulphuric  acid  (2  to  3  per  cent,  by  volume),  and  electro- 


IS2  ELECTRO-ANALYSIS. 

lyze  with  a  current  of  N.D100  =  0.1-0.2  ampere.  The 
deposit  will  be  spongy.  On  heating  to  6o°-65°  and  elec- 
trolyzing  with  N.D100  =  o.O5  ampere  and  1.2  volts,  the 
platinum  will  be  completely  precipitated  in  from  four  to 
five  hours  in  a  perfectly  adherent  form.  It  is  often  so 
dense  as  to  be  distinguished  from  hammered  platinum  with 
difficulty. 

In  the  Munich  laboratory  the  platinum  salt  solution  is 
mixed  with  2  per  cent,  (by  volume)  of  a  dilute  sulphuric 
acid  (i  :  5),  heated  to  70°,  and  electrolyzed  with  N.D100  = 
0.01-0.03  ampere.  The  precipitation  will  be  complete  in 
five  hours. 

The  following  experiment  executed  in  this  laboratory 
demonstrates  that  the  precipitation  of  platinum  from  solu- 
tions containing  sodium  phosphate  and  free  phosphoric 
acid  is  complete.  The  volume  of  the  liquid  was  150  c.c. 
It  contained  0.1144  gram  of  metallic  platinum,  30  c.c.  of 
disodium  hydrogen  phosphate  (sp.  gr.  1.0358),  and  5  c.c. 
of  phosphoric  acid  (sp.  gr.  1.347).  The  current  equaled 
0.8  ampere.  The  deposit  of  platinum  weighed  0.1140 
gram.  It  was  precipitated  upon  a  copper-coated  platinum 
dish.  It  was  washed  with  water  and  alcohol.  Ten  hours 
were  required  for  the  deposition. 

The  Rapid  Precipitation  of  Platinum  With  the  Use  of 
the  Rotating  Anode. 

In  making  the  trials  to  obtain  a  rapid  precipitation  of 
metal  a  solution  of  potassium  platinum  chloride  was  used. 
Twenty-five  cubic  centimeters  of  this  solution  contained 
0.0953  gram  of  platinum.  The  metal  was  deposited  on  a 
silver  coated  dish.  The  rotating  dish  anode  (p.  73)  was 
used  in  this  electrolysis. 


DETERMINATION    OF    METALS PALLADIUM. 


153 


No. 

H2SO4 

(DlL     I.IO) 

VOLTS 

AMPERES 

TIME, 
MIN. 

WT.  OF  PT. 
IN  GRAMS. 

IN  C  C. 

, 

5 

5 

10 

7 

0.0953 

2 

2.5 

10 

16 

3 

0.0952 

On  doubling  the  volume  of  the  solution  the  following 
results  were  obtained : 


No. 

H2S04 
(Diu  x:io) 

IN  C.C. 

VOLTS. 

AMPERES. 

TIME, 
MIN. 

WT.  OF  PT. 
IN  GRAMS. 

I 

2-5 

10 

17 

I 

0.1158 

2 

2.5 

10 

18 

2 

0.1734 

3 

2.5 

10 

16 

3 

0.1855 

4 

2.5 

10 

18 

4 

0.1903 

5 

25 

10 

17 

5 

0.1904 

The  rate  of  precipitation  is  very  evident  from  these 
figures. 

PALLADIUM. 

LITERATURE. — Wohler,  Ann.,  143,  375;  Schucht,  Z.  f.  a.  Ch.,  22, 
242;  Smith  and  Keller,  Am.  Ch.  Jr.,  12,  252;  Smith,  Am.  Ch.  Jr., 
13,  206;  14,  435;  Joly  and  Lei  die,  C.  r.,  116,  146;  Z.  f.  anorg.  Ch., 
3,  476;  Amberg,  Z.  f.  Elektrochem.,  10  (1904),  386;  Annalen,  341, 
271  ;  Langness,  J.  Am.  Chem.  S.,  29,  467. 

Palladium  can  be  deposited  from  solutions  of  the  same 
kind  and  in  the  same  manner  as  platinum.  A  bright 
metallic  deposit  will  be  obtained  by  the  use  of  a  current 
of  N.D100  =  0.05  ampere  and  1.2  volts;  otherwise  it  is 
spongy. 

It  has  been  discovered,  in  this  laboratory,  that  this 
metal  can  be  rapidly  and  fully  precipitated  from  ammoni- 
acal  solutions  of  palladammonium  chloride,  Pd(NH3Cl)2, 
which  may  be  prepared  by  adding  hydrochloric  acid  to  an 


1 54  ELECTRO-ANALYSIS. 

ammonium  hydroxide  solution  of  palladious  chloride.  To 
show  the  accuracy  of  this  method,  several  actual  determi- 
nations are  here  introduced:  (i)  A  quantity  of  the  double 
salt  (=0.2228  gram  of  palladium)  was  dissolved  in  am- 
monium hydroxide;  to  this  solution  were  added  20-30  c.c, 
of  the  same  reagent  (sp.  gr.  0.935)  anc^  IO°  c-c-  °f  water. 
A  current  of  0.07-0.1  ampere  acted  upon  this  mixture 
through  the  night,  and  deposited  0.2225  gram  of  palladium. 
(2)  In  another  experiment,  with  conditions  similar  to  those 
just  mentioned,  excepting  that  the  quantity  of  the  pallad- 
ammonium  chloride  was  doubled,  and  the  current  held  at 
0.7  ampere,  the  quantity  of  metal  precipitated  equaled 
0.4462  gram  instead  of  0.4456.  Oxide  did  not  separate 
upon  the  anode.  The  deposit,  when  dry,  showed  the  same 
appearance  as  is  ordinarily  observed  with  this  metal  in  sheet 
form.  It  was  washed  with  hot  (70°)  water,  and  dried  in 
an  air-bath  at  no0— 115°.  It  is  best  to  deposit  the  palla- 
dium in  platinum  dishes  previously  coated  with  silver. 

The  Rapid  Precipitation  of  Palladium  With  the  Use  of 
a  Rotating  Anode. 

Amberg  mentions  having  electrolyzed  palladosammine 
chloride  in  sulphuric  acid  solution  with  a  current  of  0.3 
ampere  and  1.25  volts,  when  he  succeeded  in  precipitating 
one  gram  of  palladium  upon  a  roughened  dish  in  three 
hours.  The  anode  performed  from  600  to  650  revolutions 
per  minute.  The  electrolyte  was  heated  to  65°.  The 
deposit  of  metal  was  perfectly  adherent  and  resembled 
platinum.  This  chemist  abandoned  the  silver  or  gold 
coated  platinum  cathode,  preferring  to  deposit  the  palla- 
dium directly  upon  the  platinum  from  which  he  later  dis- 
solved it  by  means  of  a  saturated  potassium  chloride  solu- 
tion (7o°-8o°)  to  which  were  added  crystals  of  chromic 


DETERMINATION    OF    METALS PALLADIUM. 


155 


acid.  This  freshly  prepared  solution  was  poured  over  the 
palladium  and  the  dish  rocked  constantly  so  that  the  plati- 
num was  only  superficially  attacked — if  affected  at  all. 

In  this  laboratory  perfectly  analogous  results  were  ob- 
tained by  electrolyzing  an  ammoniacal  solution  of  pallad- 
ammonium  chloride.  The  anode  was  the  dish  (p.  73) 
used  to  such  advantage  in  many  other  instances.  Portions 
of  such  a  solution  ( 10  cubic  centimeters  contained  0.2680 
gram  of  metal)  were  mixed  with  20  cubic  centimeters  of 
boiling  ammonium  hydroxide,  diluted  with  water  to  60  cu- 
bic centimeters  and  electrolyzed. 

RESULTS. 


No. 

VOLTS. 

AMPERES. 

TIME,  Mm. 

WT.  OF  Pd. 
IN  GRAMS. 

! 

5-6 

2  + 

1-8 

0.2682 

2 

II 

5 

10 

0.2680 

3 

17 

7 

5 

0.2682 

4 

17 

10 

3 

0.2678 

5 

17 

10 

2 

0.2678 

6 

17 

10 

2 

0.2683 

7 

17 

10 

2 

0.2680 

8 

17 

10 

2 

o  2681 

The  deposits  were  gray  in  color  and  perfectly  adherent. 
In  the  last  three  the  palladium  was  deposited  directly  on  the 
platinum  dish.  It  was  later  removed  by  the  mixture  to 
which  reference  has  been  made. 

In  a  second  series  the  quantity  of  metal  present  equaled 
in  each  instance  0.5360  gram. 

RESULTS. 


'  No. 

NH4OH  INC  c. 

DILUTION. 

VOLTS. 

AMPERES. 

TIME, 
Mm. 

WT.  OF  P. 
m  GRAMS 

I 

20 

60  c.c. 

15 

14 

3 

0.5358 

2 

20 

60  c.c. 

17 

14-20 

2 

0-5357 

3 

20 

60  c.c. 

17 

14-20 

I 

0.4966 

156  ELECTRO- ANALYSIS. 

The  deposits  were  almost  like  platinum  in  appearance. 
This  procedure  is  particularly  satisfactory  with  palladium; 
the  time  element  is  almost  annihilated. 


RHODIUM. 

LITERATURE. — Smith,  Jr.  An.  Ch.,  5,  201;  Joly  and  Lei  dip,  C.  r., 
U2,  793;  Langness,  J.  Am.  Ch.  S.,  29,  469. 

Few  attempts  have  been  made  to  determine  this  metal 
electrolytically.  Its  separation  from  an  acid  phosphate 
solution  is  very  rapid  and  complete.  A  current  of  o.iS 
ampere  will  answer  perfectly  for  the  purpose.  As  the 
decomposition  progresses,  the  beautiful  purple  color  of  the 
liquid  gradually  disappears,  and  the  solution  is  colorless 
when  the  precipitation  is  finished.  The  deposition  of  the 
rhodium  should  be  made  upon  copper-coated  dishes.  The 
metal  is  generally  black  in  color,  very  compact,  and  per- 
fectly adherent.  Hot  water  may  be  used  for  washing 
purposes. 

Joly  precipitates  the  metal  from  solutions  acidulated  with 
sulphuric  acid. 

The  Rapid  Precipitation  of  Rhodium  With  the  Use  of 
a  Rotating  Anode. 

The  electrolyte  consisted  of  an  aqueous  solution  of  rho- 
dium sodium  chloride  (0.0576  gram  of  metal)  to  which 
were  added  2.5  c.c.  of  sulphuric  acid  (dil.  i  :  10).  It 
was  diluted  to  100  c.c.  with  boiling  water,  and  electrolyzed, 
using  a  spiral  (p.  73)  anode;  while  in  the  last  three  de- 
terminations a  dish  (p.  73)  anode  was  employed.  The 
rhodium  was  deposited  on  a  silver-coated  platinum  dish. 


DETERMINATION    OF    METALS MOLYBDENUM. 


157 


No. 

VOLTS. 

AMPERES 

TIME,  MIN 

WT.  OF  RH.  IN 
GRAMS. 

I 

7 

8 

15 

0.0577 

2 

7-5 

8 

IO 

0.0580 

3 

8 

9 

10 

0.0575 

4 

8 

9 

7 

0.0576 

5 

8 

15 

4 

0.0573 

6 

6 

ii 

4 

0.0563 

7 

7 

14 

4 

0.0567 

The  deposits  were  adherent  and  black  in  color. 

The  rate  of  precipitation  was  determined  with  a  solution 
containing-  0.1153  gram  of  metal.  The  current  equaled 
15  amperes  and  the  pressure  7  volts.  The  results  were: 

In    i  minute 0.0896  gram  of  metal 

In    2  minutes 0.1006  gram  of  metal 

In    3  minutes o.i  104  gram  of  metal 

In    4  minutes 0.1128  gram  of  metal 

In    5  minutes o.i  141  gram  of  metal 

In    8  minutes 0.1152  gram  of  metal 

In  10  minutes 0.1153  gram  of  metal 


MOLYBDENUM. 

LITERATURE. — Gahn,  Gilbert's  Ann.,  14,  235;  Feree,  C.  r.,  122, 
733  ;  Smith,  Am.  Ch.  Jr.,  i,  329  ;  Hoskinson  and  Smith,  ibid.,  7,  90  ; 
Kollock  and  Smith,  J.  Am.  Ch.  S.,  23,  669;  Exner,  J.  Am.  Chem. 
S.,  25,  904;  Myers,  J.  Am.  Chem.  S.,  26,  1129;  Chilesotti,  Gazz. 
Chim.  ital.,  33,  349,  362;  Z.  f.  Elektrochem.,  12,  146;  Chilesotti  and 
Rozzi,  Gazz.  Chim.  ital.,  35  (1905),  228;  Wherry  and  Smith,  J. 
Am.  Ch.  S.,  29,  806;  Chilesotti,  Z.  f.  Elektrochem.,  12,  146. 

When  the  electric  current  acts  upon  ammoniacal  or 
feebly  acid  solutions  of  ammonium  molybdate,  a  beautiful 
iridescence  appears;  as  the  action  continues  this  assumes 
a  black  color,  and  the  deposit  becomes  more  dense.  It  is 
the  hydrated  sesquioxide  which  is  precipitated.  At  the 


158  ELECTRO-ANALYSIS. 

time  when  these  observations  were  made,  experiments  were 
instituted  to  determine  the  metal.  The  results,  while 
quantitative  in  character,  were  obtained  with  the  consump- 
tion of  too  much  time  to  permit  of  the  method  being 
generally  applied.  Recently  attention  has  again  been 
given  to  the  subject  in  this  laboratory.  Sodium  molyb- 
date  (Na2MoO4.2H2O)  was  dissolved  so  that  0.1302  gram 
of  molybdenum  trioxide  was  present  in  125  c.c.  of  solution, 
which  was  exposed  for  several  hours  to  the  action  of  a 
current  of  o.i  ampere  and  4  volts.  The  temperature  of 
the  electrolyte  was  75°  C.  No  precipitation  occurred  upon 
either  electrode.  Upon  adding  two  drops  of  concentrated 
sulphuric  acid  to  the  liquid,  it  at  once  assumed  a  dark  blue 
color.  As  the  current  continued  to  act,  this  color  dis- 
appeared and  the  cathode  was  coated  with  a  black  deposit — 
the  hydrated  sesquioxide.  On  removing  the  colorless  liquid 
and  testing  it  with  ammonium  thiocyanide,  zinc,  and  hydro- 
chloric acid,  evidences  of  the  presence  of  molybdenum 
failed  to  appear.  The  deposit  was  brilliant  black  in  color 
and  so  adherent  that  it  could  be  washed  without  detaching 
any  particles.  Usually  the  colorless  liquid  was  removed 
with  a  siphon,  cold  water  being  introduced  without  inter- 
rupting the  current.  The  deposit  was  not  dried,  but  dis- 
solved while  moist  from  off  the  dish  in  dilute  nitric  acid, 
and  the  solution  carefully  evaporated  to  dryness,  the  residue 
being  heated  upon  an  iron  plate  to  expel  the  final  traces  of 
acid.  White  molybdic  acid  remained.  If  blue  spots  ap- 
peared in  the  mass,  they  were  removed  by  moistening  the 
residue  with  nitric  acid  and  evaporating  a  second  time  to 
dryness.  This  procedure  was  adopted  in  all  the  experi- 
ments. It  was  not  possible  to  obtain  concordant  results 
by  merely  drying  the  hydrate  at  a  definite  temperature. 
The  same  was  true  in  regard  to  the  ignition  of  the  hy- 


DETERMINATION    OF    METALS- — MOLYBDENUM. 


59 


drate   to   trioxide.     Loss   occurred    from   sublimation   and 
volatilization. 

RESULTS. 


y 

U 

OS 

£  u  •* 

Z     Q       .,  [/) 

K  Q 

u 

M 

2 

D 

o 

Z   Q   ~  tfl 

n  5  ^  ^ 

S  Q    . 

•  Z 

^     • 

fri 

aTS 

s'sl'J 

3^ 

O 

g 

1° 

CURRENT. 

ij 
o 

g||j 

0  < 

as  7 

o  H  as 

C/3  U 

_) 

M 

X 

o  H  ft, 

M 

S 

Q 

H 

H 

2 

0.1302 

O.  I 

I25 

70 

N.D107=O.O22A 

2.O 

4/4 

0.1299 

—  0.0003 

0.1302 

O.I 

125 

80 

N.D107  ^=0.045  A 

2.25 

2  i/ 

0.1302 



0.1302 

O.  I 

I2S 

70 

N.D107=o.04  A 

2.2 

4  1^ 

0.1302 



o.  2604 

0.2 

125 

75 

N.D107=o.04  A 

2.O 

7 

0.2603 

—  O.OOOI 

O.I54I 

0.2 

125 

85 

N  D         o  04  A 

1.9 

2| 

0.1541 

O.I54I 

0.2 

125 

80 

N.-D107=o.o35A 

2.  I 

4 

0.1540 

—  0.0001 

The  method  is  accurate,  is  easy  of  execution,  and  re- 
quires comparatively  little  time. 

Chilesotti  and  Rozzi  have  applied  this  method  in  the 
estimation  of  molybdenum  and  have  met  with  excellent  suc- 
cess. At  first,  in  the  presence  of  alkali  metals,  they  observed 
that  these  were  carried  into  the  molybdenum  sesquioxide, 
but  subsequently  discovered  that  by  addition  of  sulphuric 
acid  any  alkali  co-precipitated  with  the  molybdenum  was 
reduced  to  nil.  In  the  presence  of  0.75  per  cent,  of  potassium 
sulphate,  0.4  per  cent,  to  0.50  per  cent,  of  sulphuric  acid 
was  sufficient  to  arrest  all  alkali  precipitation. 
^It  seemed  that  the  method  could  be  made  useful  in  the 
determination  of  the  molybdenum  content  of  the  mineral 
molybdenite.  By  fusing  the  latter  with  a  mixture  of  pure 
alkaline  carbonate  and  nitrate,  sodium  molybdate  and  sul- 
phate would  be  formed.  If  the  sulphur  is  not  to  be  deter- 
mined, after  dissolving  out  the  fusion  with  water,  and 
filtering  off  the  insoluble  oxides,  acidulate  the  alkaline 
liquid  with  dilute  sulphuric  acid  and  proceed  with  the  elec- 


i6o 


ELECTRO-ANALYSIS. 


trolysis;  but  in  cases  where  an  estimation  of  the  sulphur 
is  desired,  it  was  thought  that  acetic  acid  would  answer 
for  the  purpose  of  acidulation.  To  ascertain  the  latter 
fact  the  experiments  given  below  were  instituted.  The 
solution,  acidified  with  this  acid,  does  not  acquire  a  blue 
color  on  passing  the  current  through  it.  The  deposit  of 
hydrated  oxide  is  very  adherent  and  readily  washed.  A 
longer  time  is  necessary  for  the  complete  precipitation.  It 
is  also  advisable  not  to  add  the  entire  volume  of  acetic  acid 
at  first,  but  to  introduce  it  gradually  from  time  to  time, 
from  a  burette. 

RESULTS. 


«  °! 

U 

H 

in 

s 

£}     M     S 

S  U 

u 

Eti 

s 

o  a  ss 

^ 

D 

H 

o 

R  3  *•  as 

g  «  H  g 

ji     .  H 

£ 

5  . 

1 

S 

EC 

glil 

if" 

05  o  w  5; 

h  £  Q 

0 

^ 

»; 

o 

o  < 

2  S  u  O 

5  w^ 

H 

^ 

w 

SrS  §o 

x  O 

^^  a 

3 

i 

CJ 

S 

H 

S     ^ 

M   U 

Q 

H 

H 

S 

O.I54I 

I 

I2S 

85 

N.D107^  0.075  A 

4.4 

7^ 

0.1541 



0.1541 

I 

125 

85 

N.D1OT=  0.075  A 

44 

3 

0.1540 

—  O.OOOI 

0.1541 

I 

I25 

80 

N.D107=o.oS    A 

2-5 

6 

0.1543 

-[-O.OOO2 

In  the  last  experiment,  5  grams  of  sodium  acetate  were 
added  in  order  to  increase  the  conductivity  of  the  solution 
and  to  ascertain  what  effect  an  excess  of  this  salt  would 
have,  because,  if  the  acetic  acid  were  used  to  acidify  the 
alkaline  solution  obtained  by  the  decomposition  of  molyb- 
denite, a  great  deal  of  this  salt  would  be  present.  The 
concordant  results  justified  the  next  step,  which  was  to 
decompose  weighed  amounts  of  pulverized  molybdenite 
with  sodium  carbonate  and  nitrate,  then  take  up  the  fusion 
with  water,  filter  out  the  insoluble  oxides,  acidify  with 
acetic  acid,  boil  off  the  carbon  dioxide,  and  electrolyze. 
The  liquid  poured  off  from  the  deposit  of  the  sesquihy- 


DETERMINATION    OF    METAL' 


-MOLYBDENUM. 


161 


droxide  was  heated  to  boiling  and  precipitated  with  a  hot 
solution  of  barium  chloride. 


MOLYBDENITE, 
IN  GRAMS. 

MOLYBDENUM  FOUND, 
IN  PER  CENT. 

SULPHUR  FOUND, 
IN  PER  CENT. 

I 

2 

3 

0.2869 
0.1005 
0.1388 

57-37 
57-15 
56.83 

38.28 
38.33 
37-87 

The  Rapid.  Precipitation  of  Molybdenum  Sesquioxide 
With  the  Use  of  a  Rotating  Anode. 

The  procedure  was  the  same  as  described  under  all  the 
other  metals.  The  solutions  were  acidulated  with  sulphuric 
acid  and  the  conditions  were  as  given  here. 


a 

H 
Z     . 

W  r, 

JN 

Z 

s  M 

^ 
M  tA 

Q 
i 

o 

|| 

w  ^  u 

325 

s  £  * 
«  5  s 
w  ^  3 

H  S 

2      ^ 

M  td 

H 
J 

M 

g 

£ 

fe 

& 

~"Z 

a|| 

S53 

*JJ 

iS 

5^ 

O 

H 

o 

i 

J~ 

s 

I 

O.I  2OO 

2 

5 

16 

30 

0.1197 

2 

O.  I2OO 

2 

5 

16 

5 

0.0335 

3 

O.  I2OO 

2 

5 

16 

9 

0.0603 

4 

O.  I2OO 

2 

5 

16 

15 

o.  1026 

5 

O.  I2OO 

2 

5 

16 

20 

0.1190 

6 

O.  I2OO 

2 

5 

16 

25 

0.1198 

The  total  dilution  never  exceeded  100  cubic  centimeters. 
The  rapidity  with  which  the  oxide  separates  and  the  ease 
with  which  the  estimation  is  made  make  this  electrolytic 
procedure  vastly  superior  to  other  methods  of  determina- 
tion. 


1 62 


ELECTRO-ANALYSIS. 


The  Rapid  Precipitation  of  Molybdenum  With  the  Use 
of  a   Mercury   Cathode. 

On  electrolyzing  an  aqueous  solution  of  molybdenum 
trioxide,  acidulated  with  sulphuric  acid,  with  a  cathode  of 
mercury,  molybdenum  itself  enters  fully  into  the  cathode 
and  forms  with  it  a  brilliant  white  amalgam.  Therefore 
this  metal  can  be  directly  weighed  in  this  way.  A  water 
solution  of  sodium  molybdate,  acidulated  with  sulphuric 
acid,  will  serve  also  for  this  purpose.  Accordingly,  portions 
of  sodium  molybdate  (10  cubic  centimeters  of  which  con- 
tained 0.0950  gram  of  metal)  were  electrolyzed  under  the 
following  conditions.  The  anode  was  stationary. 

DETERMINATION  OF  MOLYBDENUM. 


<ft 
**< 

s  Si- 

Q 
\ 

Q     S2 

u^.° 

CONDITIONS. 

»  S 

w° 

lo 

S3 

H) 

^^Q 

00M 

0  * 

0  S5 

J 

a  H  * 

rf 

ri 

>•  H 

r" 

u 

B.ffe 

W  K 

i 
M 

H 

M 
M 

i 

•J  S5 

-  a 

h 

a  S  g 

%     ^ 

ID 

s 

M 

3 

O  H 

o  z 

o 

fcC/3  S 

PH 

E 

o 

SIS 

K 
OH 

sl 

6 
^ 

g™a 

«  fi 

hffi 

M 
<3 

^ 

I 
<U 

I 

O.O95O 

0.0950 

3 

13 

14 

1.2 

6 

1.6 

6.5(2  hrs.  ) 

2 

O.O95O 

0.0950 

3 

13 

22 

1.2 

6 

1.6 

6     (  2  hrs.  ) 

3 

O.I9OO 

0.1906 

2 

30 

18 

1.6 

5-5 

1.4 

7     (4  hrs.) 

4 

O.I9OO 

0.1903 

2 

25 

2O 

1.6 

5-5 

7     (4  hrs.) 

The  ordinary  steps,  observed  in  treatment  of  the  amalgam 
with  other  metals,  are  observed  here. 

This  method  of  determining  molybdenum  affords  an 
excellent  means  of  separating  it  from  other  metals  (see 
p.  272). 

GOLD. 

LITERATURE. — Luckow,  Z.  f.  a.  Ch.,  19,  14;  Brugnatelli,  Phil. 
Mag.,  21,  187;  Smith,  Am.  Ch.  Jr.,  13,  206;  Smith  and  Muhr,  Am.  Ch. 
Jr.,  13,  417;  Smith,  Jr.  An.  Ch.,  5,  204;  Smith  and  Wallace,  Ber., 


DETERMINATION    OF    METALS GOLD.  163 

25>  779J  Frankel,  Jr.  Fr.  Ins.,  1891;  Persoz,  Ann.  Chim.  Pharm.,  65, 
164;  Riidorff,  Z.  f.  ang.  Ch.,  1892,  p.  695;  Exner,  J.  Am.  Ch.  S., 
25»  9°5;  Medway,  Am.  Jr.  Science  [4th  series]  18,  58;  Per  kin  and 
Preble,  Electrochemische  Zeitschrift,  u,  69;  Mill'er,  J.  Am.  Ch. 
S.,  25,  896;  Wi  throw,  J.  Am.  Ch.  S.,  28,  1350;  J.  Am.  Ch.  S., 
27,  1545- 

This  metal  can  be  completely  deposited  from  solutions 
containing  it  in  the  form  of  a  double  cyanide,  sulphaurate, 
and  sulphocyanide,  as  well  as  in  the  presence  of  free  phos- 
phoric acid.  In  this  laboratory  the  cyanide  and  sulphaurate 
have  received  the  most  consideration.  An  example  will 
illustrate  the  conditions  with  which  good  results  may  be 
obtained  from  the  double  cyanide:  A  solution  contained 
o.i  162  gram  of  metallic  gold;  to  it  were  added  1.5  grams  of 
potassium  cyanide  and  150  c.c.  of  water.  It  was  heated  to 
55°  and  electrolyzed  with  a  current  of  N.D100  =  o.38  am- 
pere and  2.7-3.8  volts.  The  precipitation  was  complete  in 
one  and  one-half  hours.  The  gold  deposit  weighed  0.1163 
gram.  It  was  washed  both  with  cold  and  hot  water.  The 
metal  may  be  precipitated  upon  silver-coated  or  copper- 
coated  platinum  vessels,  or  directly  upon  the  sides  of  the 
platinum  dish.  If  the  last  suggestion  is  followed,  dissolve 
off  the  gold,  after  weighing,  by  introducing  very  dilute  potas- 
sium cyanide  into  the  dish,  and  then  connect  the  latter  with 
the  anode  of  a  battery  yielding  a  very  feeble  current. 

Perkin  and  Preble  dissolve  the  gold  from  off  the  platinum 
by  pouring  into  the  dish  100  c.c.  of  water  containing  two  to 
three  grams  of  potassium  cyanide  and  adding  to  this  five 
cubic  centimeters  of  hydrogen  peroxide.  In  the  cold  two  to 
three  minutes  will  be  required  for  the  solution  of  the  gold. 
One  minute  is  sufficient  if  the  solution  be  gently  heated. 

The  deposition  of  gold  from  a  sodium  sulphide  solution 
(sp.  gr.  1.18)  is  just  as  satisfactory  as  that  described  in  the 
last  paragraph.  The  current  should  equal  0.1-0.2  ampere 


164 


ELECTRO-ANALYSIS. 


for  a  total  dilution  of  about  125  c.c.     The  precipitated  metal 
is  very  adherent  and  of  a  bright  yellow  color. 

The  Rapid  Precipitation  of  Gold  With  the  Use  of  a 
Rotating  Anode. 

Use  a  double  cyanide  electrolyte  and  follow  the  condi- 
tions given  in  the  subjoined  table. 


• 

Q 

z  ^ 

*d 

61 

. 

£  u* 

R 

3  i/i 

3< 

fcS 

g  8 

5 

M  g 

£3 

S3 

So 

o 

h| 

S3 

O 

O 

0.0290 

I.O 

5 

II 

IO 

0.0289 

0.0725 

2.0 

5 

II 

II 

0.0725 

0.1450 

i-5 

5 

II 

7 

0.1447 

The  anode  should  perform  500  revolutions  per  minute. 
In  the  examples  given  the  deposits  were  excellent. 

Withrow,  in  developing  this  study,  found  the  following 
results : 


jf 

C/5 

P 

u 

% 

^T  rj) 

td   * 

•i 

8 

.1   2* 

K 

o 

§§ 

H 

011  H 

u"£ 

o  S 

O 

H  < 

O 

D  b 

|s 

J 

0 

Q  g 

a  z 

S  P 

.-  < 
M 

Q  rj 

fc 

•J 

^  s 

E~*  M 

Q  (^ 

M 

Q 

CJ^J 

C/2 

S 

0 

O 

^ 

O 

i 

0.5222 

5 

60 

IO 

io  -8 

800 

IO 

0.5216 

2 

0.5222 

5 

60 

10  -10.2 

10  -7.3 

800 

12 

0.5226 

3 

0.5222 

2-5 

55 

10  -10.8 

14.5-9.6 

800 

IO 

0.5222 

4 

0.5222 

2-5 

55 

10  -10.3 

14  -9.4 

810 

12 

0.5234 

5 

0.5465 

3-5 

60 

10  -10.5 

8.3-7 

790 

12 

0.5461 

6 

0.5465 

5 

60 

10  -10.2 

9-3  8.3 

790 

I     0.1891 

7 

0.5465 

5 

60 

10.2-10.5 

8.3-7 

800 

3    0.4341 

8 

0.5465 

5 

60 

10  -10.3 

9.6-7.1 

825 

5    0.5286 

9 

0.5465 

5 

60 

IO 

8.6-6.7 

780 

7    0.5437 

IO 

o  5465 

5 

60 

10.3-10 

8.3-6.3 

790 

ii  i  o.  5468 

ii 

0.5465 

5 

60 

16 

7.8-6.8 

790 

12 

0.5467 

DETERMINATION    OF    METALS GOLD. 


,65 


The  rate  of  precipitation  is  readily  determined  from  these 
data. 

In  an  alkaline  sulphide  electrolyte  results  may  be  obtained, 
which  are  just  as  satisfactory.  In  using  this  electrolyte 
bring  the  alkaline  sulphide  into  the  cathode  dish,  rotate  the 
anode  and  then  run  in  from  a  pipette  the  solution  of  gold 
chloride. 

RESULTS. 


B- 

CJ 

z 

M 

c/3 

z 

6 

«| 

u 
in 

0 
H  U 

W  M 

K  " 

o 

Bfi 

li 

»5 

«  z 

Ii 

Q  * 

o 
O 

$ 

Q 

« 

** 

r 

I 

0.2878 

15 

60 

10  -  8.8 

7.6-  7.2 

810 

7 

0.2891 

2 

0.2878 

30 

60 

10.1-10.3 

6.9-  6 

840 

7 

0.2879 

3 

0.2878 

30 

60 

9.8-10.1 

7.8 

830 

7 

0.2897 

4 

0.2878 

15 

60 

10  -  9.8 

11.6-n.i 

840 

7 

0.2898 

5 

0.2878 

20 

60 

10 

ii.  6-  9 

800 

7 

0.2905 

6 

0.2878 

3° 

60 

10.2-10  5 

8.8-  7.4 

830 

7 

0.2883 

7 

0.2878 

20 

60 

IO.I-IO 

9.1-  8.2 

850 

7 

0.2885 

8 

0.2878 

15 

60 

10 

11.5-10 

840 

7 

0.2887 

9 

0.2878 

30 

60 

IO.I-IO 

9-4-  8.5 

850 

0.1165 

10 

0.2878 

30 

60 

10 

8  -7 

850 

6 

0.2870 

ii  0.2878 

30 

60 

10  -10.2 

9  -  7-9 

850 

3 

0.2365 

The  Rapid  Precipitation  of  Gold  With  the  Use  of  a 
Rotating  Anode  and  Mercury  Cathode. 

Introduce  the  gold  chloride  solution  into  the  mercury  cup. 
Place  upon  it  10  cubic  centimeters  of  toluene.  Electrolyze 
with  a  current  of  from  2  to  3  amperes  and  10  volts.  The 
gold  is  precipitated  very  rapidly.  The  other  details  of 
manipulation  are  analogous  to  those  recited  under  preceding 
metals. 

Five  minutes  are  more  than  enough  to  precipitate  from 
0.15  to  0.2  gram  of  metal. 


1 66  ELECTRO-ANALYSIS. 


TIN. 

LITERATURE. —  Luckow,  Z.  f.  a.  Ch.,  19,  13;  Classen  and  v.  Reiss, 
Ber.,  14,  1622;  Gibbs,  Ch.  N.,  42,  291;  Classen,  Ber.,  17,  2467;  18, 
1104;  Bongartz  and  Classen,  Ber.,  21,  2900;  Riidorff,  Z.  f.  ang.  Ch., 
1892,  199;  Classen,  Ber.,  27,  2060;  Engels,  Z.  f.  Elektrochem.,  2,418; 
Freudenberg,  Z.  f.  ph.  Ch.,  12,  121;  Heidenreich,  Ber.,  28,  1586; 
Campbell  and  Champion,  J.  Am.  Ch.  S.,  20,  687;  Klapproth,  Dis- 
sertation, Hannover,  1901;  Classen,  Z.  £.  Elektrochem.,  i,  289; 
Henz,  Z.  f.  anorg.  Ch.,  37,  40;  Fischer  and  Boddaert,  Z.  f.  Elektro- 
chem., 10,  951;  Medway,  Am.  Jour.  Science  [4th  series],  18,  57; 
D a n n e e  1  and  Nissenson,  Internationaler  Congress  fur  angew.  Chemie 
(1903)  Band,  4,  678;  Exner,  J.  Am.  Chem.  S.,  25,  905;  Kollock 
and  Smith,  J.  Am.  Ch.  S.,  27,  1532  and  1546;  Witmer,  J.  Am.  Ch. 
S.,  29,  473. 

Tin  may  be  deposited  from  a  solution  of  ammonium  tin 
oxalate.  It  is  advisable  not  to  use  potassium  oxalate  in 
the  electrolysis,  for  then  a  basic  salt  is  liable  to  separate  upon 
the  anode. 

Classen  adds  120  c.c.  of  a  saturated  ammonium  oxalate 
solution  to  the  liquid  containing  0.9-1.0  gram  of  stannic 
ammonium  chloride,  then  electrolyzes  at  3O°-35°  with  a 
current  of  0.3-0.6  ampere  and  2.8-3.8  volts.  Acid  am- 
monium oxalate  must  be  added  from  time  to  time  if  large 
quantities  of  metal  are  to  be  precipitated.  The  tin  separates 
in  a  brilliant,  white,  adherent  form.  It  is  washed  and  dried 
in  the  usual  way.  The  time  required  for  precipitation  is 
generally  nine  hours.  This  factor,  however,  can  be  re- 
duced, as  is  evident  from  the  following  example:  Acidulate 
the  solution  containing  0.4  gram  of  tin  and  4  grams  of 
ammonium  oxalate  with  9-10  grams  of  oxalic  acid;  heat 
to  6o°-65°,  and  electrolyze  with  N.D100  =  1-1.5  amperes. 
Acetic  acid  may  replace  the  oxalic  acid.  Fusion  with  potas- 
sium acid  sulphate  will  remove  the  tin  from  the  dish. 

Henz  dissolves  the  tin  deposit  in  nitric  acid,  containing 


DETERMINATION    OF    METALS TIN.  167 

an  excess  of  oxalic  acid,  or  fills  the  dish  with  dilute  hydro- 
chloric acid  and  adds  metallic  zinc. 

Campbell  and  Champion  use  the  oxalate  method  in  deter- 
mining tin  in  its  ores.  Fuse  I  gram  of  the  ore  with  5-6 
grams  of  a  mixture  of  equal  parts  of  soda  and  sulphur  for 
an  hour  and  a  half,  at  full  red  heat.  This  is  done  in  a 
porcelain  crucible,  placed  within  a  second  crucible  of  the 
same  material.  Dissolve  the  sulphostannate  in  from  40-50 
c.c.  of  hot  water,  filter,  and  re-fuse  the  residue  as  before. 
Add  hydrochloric  acid,  to  faint  acid  reaction,  to  the  com- 
bined solutions  of  sulpho-salts.  Stannic  sulphide  will  be 
precipitated.  Boil  off  the  hydrogen  sulphide,  add  10  c.c. 
of  hydrochloric  acid  (sp.  gr.  1.20),  and  then  gradually 
introduce  2-3  grams  of  sodium  peroxide  until  a  clear  liquid 
is  obtained.  Boil  for  three  minutes,  filter  out  the  separated 
sulphur,  add  ammonia  water  to  permanent  precipitation  and 
50  c.c.  of  a  10  per  cent,  acid  ammonium  oxalate  solution. 
Electrolyze  with  a  current  of  N.D100  =  o.i  ampere  and  4 
volts.  Allow  the  current  to  act  through  the  night.  The 
deposit  will  be  light  in  color  and  very  adherent. 

Classen  has  discovered  that  a  tin  solution  containing  an 
excess  of  ammonium  sulphide,  largely  diluted  with  water, 
yields  a  quantitative  deposition  of  the  metal  when  exposed 
to  the  action  of  a  current  from  two  Bunsen  cells.  In  dilute 
sodium  or  potassium  sulphide  solution  the  tin  precipitation 
is  incomplete,  and  whenever  such  conditions  exist,  the 
sodium  or  potassium  salt  must  be  converted  into  ammonium 
sulphide.  To  this  end  the  liquid  is  mixed  with  about  25 
grams  of  ammonium  sulphate,  free  from  iron,  and  the  solu- 
tion then  carefully  warmed  in  a  covered  vessel  until  the 
evolution  of  hydrogen  sulphide  ceases ;  after  which  the 
liquid  is  heated  to  incipient  ebullition  for  fifteen  minutes. 
Allow  it  to  cool,  dissolve  any  sodium  sulphate  which  may 


1 68  ELECTRO-ANALYSIS. 

have  separated  by  the  addition  of  water,  and  electrolyze. 
The  tin  separates  in  a  gray,  dense  layer.  Wash  it  with 
water  and  alcohol.  At  times  sulphur  sets  itself  upon  the 
tin  deposit;  this  is  difficult  to  remove,  but  can  be  detached, 
after  washing  the  deposit  with  alcohol,  by  gently  applying 
a  linen  handkerchief.  Having  potassium  sulphostannate, 
Classen  considers  it  advisable  to  convert  the  tin  into  oxalate 
and  then  electrolyze.  He  employs  two  methods.  One  will 
be  given  here : — 

Decompose  the  greater  portion  of  the  sulpho-salt  with 
dilute  sulphuric  acid  (the  liquid  must  remain  alkaline)  to 
get  rid  of  most  of  the  sulphur  as  hydrogen  sulphide,  then 
oxidize  with  hydrogen  peroxide  until  the  metastannic  acid 
produced  is  pure  white  in  color.  Acidulate  with  sulphuric 
acid,  neutralize  with  ammonia  water,  and  again  add  hydro- 
gen peroxide.  Filter  out  the  stannic  acid  when  it  has  sub- 
sided, dissolve  in  oxalic  acid  and  ammonium  oxalate,  and 
electrolyze  with  the  conditions  given  in  the  preceding  para- 
graphs. 

According  to  Carl  Engels  add  0.3  to  0.5  gram  of  hy- 
droxylamine  hydrochloride  or  sulphate,  2  grams  of  ammo- 
nium acetate,  and  2  grams  of  tartaric  acid  to  the  solution 
of  the  tin  salt,  dilute  with  water  to  150  c.c.,  heat  to  6o°-7O°, 
and  electrolyze.  with  N.D100  =  i  ampere. 

The  Rapid  Precipitation  of  Tin  With  the  Use  of  a 
Rotating  Anode. 

In  this  laboratory  no  difficulty  was  experienced  in  using 
a  solution  of  stannous  ammonium  chloride  containing  an 
excess  of  a  hot  saturated  solution  of  ammonium  oxalate. 
The  anode  performed  300  revolutions  per  minute.  The 
proper  conditions  are  shown  in  a  few  examples  which  fol- 
low : — 


DETERMINATION    OF    METALS TIN. 


169 


AMMONIUM 

TIN  PRESENT 
IN  GRAMS. 

OXALATE  HOT, 
SATURATED 
SOLUTION 

CURRENT 
N.  D.100  IN 
AMPERES. 

VOLTS. 

TIME. 
MINUTES. 

FOUND  TIN 
IN  GRAMS. 

IN  C.C. 

0.5396 

100 

5 

5 

13 

0-5392 

0.2193 

100 

5 

5-5 

15 

0.2193 

0-4355 

100 

5-8 

5-5-6-5 

1  8 

0-4353 

1.0800 

IOO 

5 

4-5 

20 

I.oSoi 

In  using  an  ammonium  sulphide  electrolyte  a  definite 
volume  of  the  alkaline  sulphide  was  placed  in  the  cathode 
dish  and  the  solution  of  stannous  chloride  pipetted  into  it. 
Hot  water  was  then  added  to  give  100  cubic  centimeters 
volume  to  the  liquid.  The  anode  was  made  to  rotate  500 
times  per  minute,  the  dish  was  covered  and  the  current  ap- 
plied. The  conditions  are  exhibited  in  the  following  experi- 
ments : 


AMMONIUM 
SULPHIDE 
(Sp.  GR.  0.985). 

CURRENT 
N  I).  ,00  IN 
AMPERES. 

VOLTS. 

TIME  IN 
MINUTES. 

TIN  PRESENT 
IN  GRAMS. 

TIN    FOUND 
IN  GRAMS. 

An  excess. 

5-4 

7 

10 

0-1357 

0.1052 

«         « 

4 

7-5 

20 

0.1357 

0.1350 

«         « 

4 

7-5 

20 

o  1357 

0.1354 

7  c.c. 

4-5 

8 

25 

0.1357 

0.1358 

H    ' 

5-4 

7-5 

25 

0.2714 

0.2717 

The  deposits  were  like  polished  silver.  When  stannic 
chloride  was  the  salt  used,  the  metal  deposit  was  slightly 
crystalline  but  perfectly  adherent.  The  speed  of  rotation 
of  the  anode  had  little  or  no  effect  on  the  character  of  the 
deposit. 

The  best  conditions  for  0.2  gram  of  metal  were  found  to 
be  15  to  20  cubic  centimeters  of  ammonium  sulphide  (sp. 
gr.  0.985)  and  a  current  of  N.D100  =  5.5  amperes  and  9 
volts. 
16 


I  ;o 


ELECTRO-ANALYSIS. 


The  rate  of  precipitation  was  determined  with  a  solution 
containing  0.5070  gram  of  metal.     It  was  found  to  be: — 

In    i  minute  0.0704  gram 

In    2  minutes 0.1276  gram 

In    3  minutes 0.1922  gram 

In    4  minutes 0.2475  gram 

In    5  minutes 0.2927  gram 

In  i  o  minutes 0.4796  gram 

In  1 5  minutes 0.49 1 7  gram 

In  20  minutes 0.5070  gram 

The  current  in  these  trials  was  N.D100  =  5  amperes  and  7.5 
to  10  volts. 

The  Rapid  Precipitation  of  Tin  With  the  Use  of  .a 

Rotating  Anode  and  Mercury  Cathode. 
Arrange  the  mercury  cup  as  under  the  preceding  metals. 
Introduce  into  it  the  tin  salt,  preferably  the  sulphate  (  5  cubic 
centimeters  =  0.4 1 06  gram),  add  a  little  concentrated  sul- 
phuric acid  and  electrolyze  with  a  current  of  from  2  to  4 
amperes  and  5  to  4  volts.  Conditions  almost  analogous  to 
these  are  found  in  the  following  examples.  They  are  re- 
liable and  give  results  that  are  dependable. 


ri 

H 

u  u 

Q 

M 

W     • 

w 

K   U 

X    • 

11 

[5 

gi 

Z     . 

D  g 

o  < 

!l 

• 
a. 
X 

z 

1" 

£  Q 

11 

o 

*=! 

—    K 

2° 

w 

H 

H 

I 

0.4106 

5 

0.2 

2-4 

5 

10 

0.4109 

-f  0.0003 

2 

0.4106 

5 

0.2 

4 

5 

9 

0.4114 

.-f  0.0008 

3 

0.4106 

5 

0.2 

4 

5-4-5 

9 

0.4109 

+0.0003 

o  4106 

6 

O.  !» 

6 

0.4106 

5 

0.4106 

5 

0.25 

.' 

4 

5 

6 

0.4106 



6 

0.8212 

10 

6 

5-5 

9 

0.8210 

—  O.OOO2 

7 

0.4106 

10 

0.75 

5 

5 

8 

0.4107 

+  O.OOOI 

8 

0.4106 

7 

0.05 

5 

5 

7 

0.4106 



9 

0.4106 

7 

0.25 

5 

5 

10 

0.4107 

+  0.0001 

DETERMINATION    OF    METALS ANTIMONY.  I /I 

The  rate  of  precipitation  is : 

In  2  minutes 0.2997  gram  of  tin 

In  4  minutes 0.3974  gram  of  tin 

In  5  minutes 0.4060  gram  of  tin 

In  6  minutes 0.4106  gram  of  tin 

On  using  a  current  of  5  amperes  and  5  to  4  volts,  0.8212 
gram  of  tin  was  precipitated  in  eight  minutes. 

Stannous  chloride  may  also  be  used  as  the  electrolyte  if 
the  layer  of  toluene  (p.  89)  is  placed  over  it.  To  illustrate, 
the  following  examples  may  be  cited : 

1.  Five  cubic  centimeters  of  stannous  chloride  (=0.0800 
gram  of  tin)  and  10  cubic  centimeters  of  toluene  were  elec- 
trolyzed  with  a  current  of  2  to  3  amperes  and  7  to  6  volts. 
In  ten  minutes  (a)  0.0798  gram  and  (b)  0.0806  gram  of 
metal  were  precipitated. 

2.  Ten  cubic  centimeters  of  stannous  chloride  (  —  0.1600 
gram  of  tin)    and  ten  cubic  centimeters  of  toluene  were 
electrolyzed  with  a  current  of  2  to  3  amperes  and  7  to  6 
volts.     In  fifteen  minutes  0.1595  and  0.1600  gram  of  metal 
were  obtained. 

ANTIMONY. 

LITERATURE. — Wrightson,  Z.  f.  a.  Ch.,  15,  300;  Parodi  and  Mas- 
cazzini,  Z.  f.  a.  Ch.,  18,  588;  Luckow,  Z.  f.  a.  Ch.,  19,  13;  Classen 
and  v.  Reiss,  Ber.,  14,  1622;  17,  2467;  18,  1104;  Lecrenier,  Ch.  Z., 
13,  1219;  Chittenden,  Pro.  Conn.  Acad.  Sci.,  Vol.  8;  Vortmann, 
Ber.,  24,  2762;  Riidorff,  Z.  f.  a.  Ch.,  1892,  199;  Classen,  Ber.,  27, 
2060;  Henz,  Z.  f.  anorg.  Ch.,  37,  29;  Ost  and  Klapproth,  Z.  f.  ang. 
Ch.  (1900),  827;  Ho  Hard,  B.  Soc.  Chim.  [series  3],  29,  262  and  C. 
N.,  87,  282;  Fischer,  Ber.,  36,  2348;  Z.  fur  anorg.  Ch.,  42,  363; 
Law  and  Per  kin,  Trans.  Faraday  Society  (1905),  i,  262;  Danneel 
and  Nissenson,  Internationaler  Congress  fur  angewandte  Ch.  (1903), 
Band  4,  678;  Exner,  J.  Am.  Ch.  S.,  25,  905;  Fischer  and  Bod- 
daert,  Z.  f.  Elektrochem.,  10,  950;  Langness  and  Smith,  J.  Am. 
Ch.  S.,  27,  1524;  Dormaar,  Z.  f.  anorg.  Ch.,  53,  349;  Foerster 
and  Wolf,  Z.  f.  Elektrochem.,  13,  205;  Sand,  Z.  f.  Elektrochem.,  13,  326. 


172  ELECTRO-ANALYSIS. 

Antimony,  when  precipitated  from  a  solution  of  its 
chloride,  or  from  that  of  antimony  potassium  oxalate,  does 
not  adhere  well  to  the  cathode.  It  is  deposited  very  slowly 
from  a  solution  of  potassium  antimony  1  tartrate.  Its  de- 
position from  a  cold  ammonium  sulphide  solution  is  satis- 
factory, but  the  use  of  this  reagent  for  this  purpose  is  not 
pleasant,  especially  when  several  analyses  are  being  carried 
out  simultaneously.  For  this  reason  potassium  or  sodium 
sulphide  has  been  substituted.  The  alkaline  sulphide  used 
must  not  contain  iron  or  alumina. 

The  antimony  solution  mixed  with  80  c.c.  of  sodium 
sulphide  (sp.  gr.  1.13—1.15),  should  be  diluted  with  water 
to  125  c.c.  and  acted  upon  at  6o°— 65°  with  a  current  of 
N.D100=  i  ampere  and  1.1-1.7  volts.  The  metal  will  be 
fully  precipitated  in  two  hours.  The  deposit  should  be 
treated  in  the  usual  way  with  water  and  pure  alcohol. 
Dry  at  90°.  To  ascertain  when  all  of  the  metal  has  been 
deposited,  incline  the  dish  slightly,  thus  exposing  a  clean 
platinum  surface.  If  this  remains  bright  for  half  an  hour 
the  precipitation  is  finished.  In  separating  antimony  from 
the  heavy  metals — e.  g.,  lead — it  happens  that  alkaline  sul- 
phides containing  polysulphides  are  used,  or  are  produced. 
To  remove  these  Classen  proposed  adding  to  the  antimony 
polysulphide  mixture,  already  in  a  weighed  platinum  dish, 
an  ammoniacal  solution  of  hydrogen  peroxide,  and  warming 
the  same  until  the  liquid  becomes  colorless.  When  this 
is  accomplished,  even  if  a  precipitate  has  been  produced, 
add,  after  cooling,  the  solution  of  sodium  monosulphide, 
and  electrolyze  as  previously  directed. 

Lecrenier  writes  as  follows  relative  to  the  preceding 
method:  The  precipitation  is  all  that  one  can  desire,  pro- 
viding the  solution  of  the  sulpho-salt  is  absolutely  free 
from  polysulphides;  otherwise,  it  is  incomplete.  The  anti- 


DETERMINATION    OF    METALS ANTIMONY.  173 

mony  sulphide  obtained  in  the  ordinary  course  of  analysis 
always  contains  sulphur,  and  this  must  be  eliminated.  To 
remove  the  various  inconveniences  connected  with  the 
method  add  50-70  c.c.  of  a  25  per  cent,  solution  of  sodium 
sulphite  to  the  solution  after  the  addition  of  the  excess  of 
sodium  sulphide,  then  heat  the  liquid  to  complete  decoloriza- 
tion;  allow  to  cool,  after  which  the  current  is  conducted 
through  the  liquid.  This  can  rise  to  0.5  ampere  without 
impairing  the  result;  but  it  is  not  best,  as  the  precipitated 
metal  is  then  very  coherent.  It  is  better  to  use  a  current 
of  0.25  ampere.  When  the  quantity  of  antimony  does  not 
exceed  0.2  gram,  the  deposit  will  be  adherent  and  free 
from  sulphur;  wash  with  water,  alcohol,  and  ether.  Sul- 
phur will  separate  upon  the  anode,  despite  the  presence  of 
an  excess  of  sodium  sulphite.  This,  however,  does  not 
affect  the  result. 

The  method  of  Classen  suffers  in  several  points : 

1.  The  bath  pressure  falls  as  the  electrolysis  proceeds, 
because  of  the  accumulation  in  it  of  sodium  polysulphide. 

2.  If  the  electrolysis   is   not  interrupted   at  the  proper 
moment,  antimony  already  precipitated  will  be  again  dis- 
solved by  the  polysulphide  which  has  diffused  toward  the 
cathode  (Z.  f.  ang.  Ch.,  1897,  325).     Ost  and  Klapproth 
have  sought  by   the  use  of   a   diaphragm   to   circumvent 
these  objectionable  features.     To  this  end  they  use   (Fig. 
30)    a  roughened  dish,  a,  in  which  is  suspended  a  dish- 
shaped  diaphragm,  b  (a  Pukall  porous  cup,  Ber.,  26,  1159). 
A  strip  of  platinum,  c,  within  the  diaphragm,  is  the  anode, 
while    the    platinum    dish    itself    constitutes   the    cathode. 
Cover-glasses   are   placed   over  both   dishes.     The   liquids 
experimented    upon    were    a    solution    of    Schlippe's    salt 
(=0.0985  gram  of  antimony  in  10  c.c.)  and  a  solution  of 
pure    sodium    sulphide    (195    grams    Na2S  =  200    grams 


ELECTRO-ANALYSIS. 


NaOH  to  the  liter).  In  the  first  experiments  the  anti- 
mony was  equally  distributed  in  the  whole  electrolyte. 
The  cathode  chamber  contained  85  c.c.  and  the  anode 

FIG.  30. 


chamber  40  c.c.  of  the  solution,  which  had  0.0985  gram  of 
antimony  in  125  c.c.,  with  varying  amounts  of  sodium 
sulphide.  The  liquid  covered  about  100  sq.  cm.  of  the 
surface  of  the  dish : 


BATH  PRESSURE  AT 

CURRENT  STRENTH 

EXPERI- 

Na2S 

TEMPER-A- 

ONE  AMPERE. 

IN  AMPERES. 

ANTIMONY 

MENT. 

TION. 

TURE. 

BEGINNING 

END 

AT 

AT 

TATED. 

VOLTS. 

VOLTS. 

BEGINNING. 

END. 

I 

5  c.c. 

70° 

3-8 

3-9 

0.7 

0-3 

0.0675 

2 

50  - 

Cold. 

1.9 

3-8 

o-5 

0.4 

0.0725 

3 

80  « 

70° 

2-5 

i-7 

I.O 

I.O 

0.0685 

4 

80  " 

70° 

i-7 

i-3 

I.O 

I.O 

0.0720 

When  the  electrolysis  was  finished,  antimony  could  not 
be  found  in  the  cathode  liquid  from  any  one  of  the  four 
experiments,  whereas  in  the  anode  chamber  it  was  still  in 
solution,  and  in  experiment  I  it  had  been  precipitated  on 
the  anode  in  the  form  of  antimony  pentasulphide. 


DETERMINATION    OF    METALS ANTIMONY. 


175 


These  experiments  indicated  then  that  the  current  is 
not  able  to  carry  antimony  ions  from  the  anode  into  the 
cathode  chamber. 

In  the  next  series  of  experiments  the  10  c.c.  of  antimony 
solution  (=0.0985  gram  of  metal)  were  placed  in  the 
cathode  chamber  alone : 


BATH  PRESSURE  AT  ONE 

EXPERI- 

Na2S 
SOLU- 

TEMPERA- 

AMPERE. 

ANTIMONY 

MENT 

TION. 

TURE. 

BEGINNING 

AT  END 

TATED. 

VOLTS. 

VOLTS. 

I 

50  c.c. 

Cold. 

4.2 

3-7 

5  hours. 

0.0970 

2 

50  c.c. 

70° 

2.O 

3-8 

3      " 

0.0984 

Temp.  32° 

3 

80  c.c. 

70° 

2-5 

1.7 

2         " 

0.0990 

4 

50  c.c. 

70° 

1.8 

1.8 

iK" 

0.0990 

The  results  show  a  quantitative  precipitation  of  the  anti- 
mony. None  of  it  could  be  found  either  in  the  cathode  or 
anode  liquid. 

On  placing  the  antimony  in  the  anode  chamber  alone, 
not  a  particle  of  metal  was  deposited  on  the  cathode. 

When  the  antimony  was  placed  in  the  cathode  chamber 
only  and  varying  quantities  of  sodium  sulphide  solution 
were  mixed  with  it,  remarkable  differences  were  observed. 
In  the  presence  of  much  sodium  sulphide  and  accompany- 
ing low  bath  pressure  all  of  the  antimony  was  precipitated 
at  the  cathode,  while  with  little  sodium  sulphide  and  con- 
sequent high  bath  pressure,  a  small  amount  of  antimony 
wandered  through  the  diaphragm  and  was  deposited  at  the 
anode  in  the  form  of  antimony  sulphide. 

These  experiments  show  how  a  successful  antimony  de- 
termination may  be  made.  No  difficulties  attend  its  esti- 
mation in  this  way. 


1 76  ELECTRO-ANALYSIS. 

To  dissolve  the  antimony  deposit  from  off  the  dish,  Ost 
recommends  nitric  acid,  containing  tartaric  acid. 

Vortmann,  recognizing  the  fact  that  it  is  difficult  to 
obtain  an  adherent  deposit  of  antimony  when  the  quantity 
of  metal  in  solution  exceeds  0.16  gram,  has  combined  the 
method  of  Smith,  who  first  pointed  out  that  mercury  could 
be  deposited  very  satisfactorily  from  its  solution  in  sodium 
sulphide,  with  his  knowledge  that  antimony  could  be  pre- 
cipitated from  a  similar  solution,  and  hence  recommends 
the  determination  of  the  antimony  in  the  form  of  an  amal- 
gam. No  difficulties  attend  this  procedure.  Two  parts 
of  mercury  should  be  present  for  every  part  of  antimony. 
The  latter  must  also  be  present  in  solution  as  higher  oxide ; 
to  this  end  digest  the  antimonious  solution  with  bromine 
water,  and  afterward  add  the  sodium  sulphide  containing 
sodium  hydroxide.  Electrolyze  with  a  current  of  from 
0.2  to  0.3  ampere.  The  amalgam  can  be  washed  in  the 
usual  way. 

Law  and  Perkin  recommend  precipitating  antimony  from 
an  ammoniacal  solution  of  its  tartrate.  To  this  end  they 
heat  the  electrolyte  to  75°  and  act  upon  it  with  a  current 
of  N.D100  =  o.2  to  0.5  ampere  and  2.5  to  3  volts. 

Almost  every  analyst  has  experienced  at  the  out-start, 
difficulties  similar  to  those  described  and  many  have  made 
suggestions  of  value  to  escape  them.  Thus,  Henz,  recog- 
nizing the  virtue  of  the  methods  adopted  by  Lecrenier  and 
Ost  and  Klapproth  to  get  rid  of  the  disturbing  influences 
due  to  the  polysulphide,  found  an  excellent  reducing  agent 
in  potassium  cyanide.  Hollard  (1900),  however,  was  the 
first  to  use  this  reagent,  antedating  Henz,  Fischer  and 
Exner.  Potassium  cyanide  rapidly  reduces  polysulphides 
to  monosulphide,  forming  a  sulphocyanide : 

KCN  +  Na2S2  =  KCNS  +  Na2S. 


DETERMINATION    OF    METALS ANTIMONY. 


177 


In  this  respect  one  gram  of  potassium  cyanide  will  be 
as  effective  as  four  grams  of  sodium  sulphite.  It  is  also 
much  more  soluble.  One  to  two  grams  will  suffice  to 
keep  colorless  the  bath  for  the  precipitation  of  o.i  gram  of 
antimony. 

While  Henz  obtained  most  satisfactory  deposits  of  anti- 
mony in  this  way  he  observed — as  have  others — that  often 
the  results  were  high;  in  some  instances  from  2  to  3  per 
cent.  He  thought  possibly  there  was  here  a  constant  for 
which  allowance  could  be  made.  Dormaar  has  since  given 
this  point  very  careful  study  and  found  that  the  apparent 
increase  in  the  found  antimony,  rising  with  the  current 
strength  and  the  quantity  of  metal  present,  is  due  in  large 
part  to  the  presence  of  oxygen  in  the  deposit  and  some 
occluded  sodium  sulphide. 

It  is  probable  that  working  with  from  o.i  to  0.2  gram  of 
metal  this  oxidation  has  been  too  slight  to  affect  the  final 
result,  so  it  has  been  usually  neglected.  ' 

The  Rapid  Precipitation  of  Antimony  With  the  Use  of 
a  Rotating  Anode. 

Exner,  working  in  this  laboratory,  first  performed  this 
determination.  He  added  to  a  solution  of  antimony  chlo- 
ride a  slight  excess  of  sodium  hydroxide,  sodium  hydro- 
sulphide  and  potassium  cyanide,  then  electrolyzed  with  con- 
ditions like  those  given  below. 


SbCl3 
EQUAL  TO 
ANTIMONY 
IN  GKAMS. 

NaOH 

10$  SOLU- 
TION INC.C. 

NaSH 
c.c. 

2O 

KCN 
GRAMS 

CURRENT 
N.D100  = 
AMPERES. 

VOLTS. 

TIME  IN 
MINUTES. 

Sb. 

0.3042 

30 

2 

5 

4-5 

2O 

0.3042 

178  ELECTRO-ANALYSIS. 

The  anode  made  400  to  500  revolutions  per  minute. 

Later  Miss  Langness  proceeded  as  follows  in  applying 
the  above  procedure.  To  a  solution  of  antimony  chloride 
(=0.2405  gram  of  metal)  were  added  15  cubic  centi- 
meters of  sodium  sulphide  (sp,  gr.  1.18),  3  grams  of  po- 
tassium cyanide,  i  cubic  centimeter  of  sodium  hydroxide 
(10  per  cent.),  the  solution  was  diluted  with  water  to  70 
cubic  centimeters,  heated  nearly  to  boiling  and  electrolyzed 
with  N.D100  =  6  amperes  and  3.5  to  4  volts.  The  metal 
was  all  deposited  in  fifteen  minutes.  Numerous  determi- 
nations were  made.  The  deposits  in  all  of  them  were  per- 
fectly adherent.  There  was  no  sponginess.  The  metal 
was  bright  gray  in  color.  On  using  sand-blasted  platinum 
dishes  from  0.4847  gram  to  i.oooo  gram  of  metal  could  be 
precipitated  in  a  beautiful  and  very  compact  form  in  from 
twenty  to  twenty-five  minutes. 

The  rate  of  precipitation,  determined  with  a  current  of 
6.5  amperes  and  3.5  volts,  was  as  follows: 

In    i   minute 0.0652  gram  of  antimony  was  obtained 

In    2  minutes 0.1007  gram  of  antimony  was  obtained 

In    3   minutes ..0.1575  gram  of  antimony  was  obtained 

In    4  minutes 0.1969  gram  of  antimony  was  obtained 

In    5   minutes 0.2140  gram  of  antimony  was  obtained 

In    6  minutes 0.2251  gram  of  antimony  was  obtained 

In    7  minutes ....0.2331  gram  of  antimony  was  obtained 

In    8  minutes 0.2369  gram  of  antimony  was  obtained 

In  15   minutes 0.2405  gram  of  antimony  was  obtained 

The  omission  of  the  sodium  hydroxide  from  the  electro- 
lyte works  no  harm.  It  is  possible  also  to  reduce  the  volume 
of  sulphide  to  ten  cubic  centimeters,  but  there  should  then 
be  a  reduction  of  the  alkaline  cyanide  to  2  grams.  The 
reduction  of  the  latter  without  a  corresponding  reduction 
of  sulphide  is  apt  to  alter  somewhat  the  character  of  the 
deposit. 


DETERMINATION    OF    METALS TELLURIUM. 

This  method  was  tried  out  under  the  most  varied  con- 
ditions, and  then  applied  to  the  mineral  stibnite.  Very 
pure  samples  of  the  latter  were  reduced  to  powder  and  0.5 
gram  portions  digested  with  20  cubic  centimeters  or  more 
of  sodium  sulphide  (1.18  sp.  gr.),  filtered  from  the  insoluble 
part,  and  after  the  addition  of  3  grams  of  potassium  cyanide 
and  one  cubic  centimeter  of  sodium  hydroxide  (10  per 
cent.),  heated  to  boiling  and  electrolyzed  with  N.D100=7 
amperes  and  3  volts.  The  results  were  perfectly  satis- 
factory. The  time  required  to  precipitate  all  the  antimony 
did  not  exceed  twenty-five  minutes.  See  also  separation 
of  antimony  from  arsenic  (p.  251). 


TELLURIUM. 

LITERATURE. — Pellini,  Gaz.  chim.  ital.,  34  (I.)  128;  Gallo,  Gaz.  chim. 
ital.,  34  (II.)  404-409;  Gallo  (Atti  R.  Accad.  dei  Lincei  Roma  [5] 
I3>  [*]  7i3;  Gazz.  chim.  ital.,  35,  514  (1905);  Schucht,  Ch.  Z. 
(1880),  292,  374;  Jahresb.  1880,  p.  174,  1143;  Schucht,  Ch.  N.,  41, 
280;  Jahresb.  (1880)  1143,  1144;  Schucht,  Z.  f.  analyt.  Ch.,  22  (1883) 
495  ;  Whitehead,  J.  Am.  Ch.  S.,  17,  849  ;  Ch.  N.,  82,  203. 

Dissolve  the  tellurium  in  nitric  acid  and  evaporate.  Heat 
the  residue  on  a  water  bath  after  the  addition  of  ten  cubic 
centimeters  of  sulphuric  acid,  introduce  30-40  cubic  centi- 
meters of  a  saturated  solution  of  acid  ammonium  tartrate 
to  complete  solution,  dilute  with  water  to  250  cubic  centi- 
meters, rotate  the  anode  at  the  rate  of  800  to  900  revo- 
lutions per  minute  and  electrolyze  with  N.D100  — 0.12  to 
0.09  ampere  and  1.8  to  1.2  volts.  The  electrolyte  should 
be  heated  to  60°  C.  Wash  the  deposit  promptly  with  water 
free  from  oxygen,  then  with  alcohol  and  dry  at  about  90° 
C.  Rather  large  quantities  of  tellurium  can  be  precipitated 
in  this  way. 


ISO  ELECTRO-ANALYSIS. 

Gallo  recommends  dissolving  distilled  tellurium  in  sul- 
phuric acid,  using  a  sand-blasted  dish,  then  evaporating  to 
the  appearance  of  white  fumes.  The  tellurium  dissolves 
as  tellurous  acid.  When  cold  add  several  cubic  centimeters 
of  boiled  water,  free  from  carbon  dioxide,  to  the  white 
residue,  dilute  to  150  cubic  centimeters  with  a  ten  per  cent, 
solution  of  sodium  or  potassium  pyrophosphate.  Heat 
gradually  to  60°  C.,  use  a  spiral  anode,  and  electrolyze  with 
a  current  of  N.D100  =  0.025  ampere  and  1.8  to  2  volts. 
About  twenty-five  milligrams  of  tellurium  will  be  precipi- 
tated per  hour. 

ARSENIC. 

LITERATURE. — Luckow,  Z.  f.  a.  Ch.,  19,  14;  Classen  and  v.  Reiss, 
Ber.,  14,  1622;  Moore,  Ch.  N.,  53,  209;  Vortmann,  Ber.,  24,  2764; 
Schulze,  Inaugural  Dissertation,  Berlin  (1900);  Thorpe,  Jr.  Ch.  Soc., 
London,  83,  974;  Sand  and  Hackford,  Jr.  Chem.  Soc.  London  (1904), 
1018;  Mai  and  Hurt,  Ch.  Z.,  29,  Heft  20  (1905),  Z.  f.  Untersuch. 
Nahr.  Genusen.  9,  193  to  199;  Frerichs  and  Rodenberg,  Arch,  der 
Pharmacie,  243,  348;  Thorpe,  Ch.  N.,  88,  7;  Trotman,  Jr.  Chem. 
Society  23,  177. 

A  successful  method  for  the  complete  deposition  of  arsenic 
is  not  known.  The  current  acting  upon  the  chloride  causes 
complete  volatilization  of  the  metal  in  the  form  of  arsine. 
Its  separation  from  oxalate  solutions  is  incomplete;  nor  do 
the  sulpho-salts  answer  for  electrolytic  purposes. 

From  a  solution  containing  0.2662  gram  of  arsenious 
oxide  Vortmann  obtained  0.18527  gram  of  metallic  arsenic, 
equivalent  to  69.59  Per  cent-  The  trioxide  contains  75.78 
per  cent,  of  arsenic.  This  precipitation  was  effected  by  the 
amalgam  method. 

The  facts  relating  to  the  electrolytic  behavior  of  vana- 
dium (Truchot,  Ann.  Chim.  Anal.  (1902),  7,  165)  tungs- 


SEPARATION    OF    METALS COPPER.  l8l 

ten,  and  osmium  are,  at  the  present  writing,  few  in  number 
and  will  not  be  introduced  here. 


2.  SEPARATION  OF  THE  METALS. 

Electrolysis  to  be  of  value,  must  not  only  furnish  the 
analyst  with  methods  suitable  for  the  complete  deposition 
of  metals,  but  it  should,  in  addition,  enable  him  to  separate 
metallic  mixtures.  The  data  given  in  the  preceding  pages 
will  serve  for  this  purpose,  but,  as  a  special  treatment  is 
required  in  some  instances,  a  brief  outline  of  a  series  of 
separations  will  be  indicated. 

It  will  be  noticed  that  the  electrolytes  vary.  The  mineral 
acid  and  the  double  cyanide  solutions  are  best  adapted  for 
the  purpose.  The  greatest  number  of  separations  have 
been  made  by  means  of  them.  Some  of  the  organic  acids, 
too,  answer  quite  well  as  will  be  seen  in  the  succeeding 
paragraphs. 

COPPER. 

Inasmuch  as  the  electrolytic  precipitation  of  copper  gives 
the  analyst  such  an  excellent  means  of  determining  this 
metal  quantitatively,  its  separations  from  other  metals  are 
of  prime  importance.  Such  separations,  so  far  as  they  have 
been  carefully  worked  out  in  the  most  essential  points,  are 
given  in  detail  in  the  following  paragraphs.  It  is  needless 
to  add  that  acid  solutions  mainly  are  best  adapted  for  these 
separations. 

i.  From  Aluminium: — 

(a)  In  nitric  acid  solution.  Dilution,  200  c.c.;  5  c.c.  of 
nitric  acid  (sp.  gr.  1.30) ;  temperature,  32°;  N.D100  = 
i  ampere  and  3.3  volts;  time,  4  hours. 


1 82  ELECTRO-ANALYSIS. 

With  a  rotating  anode.  Arrange  the  apparatus  as 
described  on  p.  72.  Dilute  the  solution  to  125  c.c., 
add  i  c.c.  of  nitric  acid  (sp.  gr.  1.43)  and  electrolyze 
with  a  current  of  N.D100  =  3  amperes  and  a  pressure 
of  4  to  5  volts.  The  anode  should  perform  300  to  400 
revolutions  per  minute.  The  time  allowed  the  precip- 
itation should  not  exceed  twenty  minutes.  Copper 
present  0.2874  gram  and  aluminium  0.2500  gram.  The 
copper  found  equaled  (a)  0.2873  gram,  (b)  0.2874 
gram  and  (c)  0.2874  gram.  J.  Am.  Ch.  S.,  26, 
1284. 

(b)  In  sulphuric  acid  solution.  Dilution,  150  c.c. ;  3  c.c. 
of  concentrated  sulphuric  acid;  temperature,  59°; 
N.D100  =.  i  ampere  and  2.5  volts;  time,  2  hours. 

With  a  rotating  anode.  With  apparatus  arranged 
as  given  on  p.  72  introduce  the  solution  of  salts  of  the 
two  metals  into  a  dish,  dilute  to  125  c.c.,  add  i  c.c.  of 
sulphuric  acid  (sp.  gr.  -1.83)  and  electrolyze  with  a  cur- 
rent of  N.D100  =  4  to  5  amperes  and  a  pressure  of  14 
to  8  volts.  Time  ten  minutes.  With  a  mercury  cath- 
ode and  rotating  anode.  This  separation  was  accom- 
plished in  the  presence  of  0.5  cubic  centimeters  of  sul- 
phuric acid  (i.i),  when  the  current  registered  i 
ampere  and  4  volts.  In  four  minutes  the  solution  was 
colorless.  The  current  was  allowed  to  act  for  ten 
minutes. 

Volume    of    the    solution        =  10   cubic   centimeters. 

Copper  sulphate  00.1150  gram  copper. 

Aluminium    sulphate  O  o.i    gram    aluminium. 

Sulphuric  acid   (i.i)  =0.5    cubic    centimeter. 

Current  =  1-1.6  ampere. 

Pressure  =4-4.5   volts. 

Time  —  10    minutes. 

Copper  found  —  0.1150  gram,  0.1153  gram,  0.1152  gram. 


SEPARATION    OF    METALS COPPER.  183 

(c)  In  phosphoric  acid  solution.  Dilution,  225  c.c. ;  5 
c.c.  of  phosphoric  acid  (sp.  gr.  1.347)  ;  temperature, 
77°  C. ;  N.D100  ==  0.068  ampere  and  2.6  volts;  time,  6 
hours.  Sixty  cubic  centimeters  of  disodium  hydro- 
gen phosphate  (sp.  gr.  1,0338)  were  present  for  0.1239 
gram  of  copper  and  o.iooo  gram  of  aluminium.  The 
precipitated  copper  weighed  0.1240  gram  (J.  Am. 
Ch.  S.,  21,  1002). 

In  this  electrolyte  the  separation  with  the  aid  of  a 
rotating  anode  is  also  possible  when  observing  these 
conditions:  Dilution  125  c.c.,  with  10  c.c.  of  phosphoric 
acid  (sp.  gr.  1.085),  5°  c-c-  °f  a  IO  Per  cent-  solution 
of  disodium  hydrogen  phosphate,  and  a  current  of 
N.D100  =  5  amperes  and  6  volts.  Time  10  minutes. 
A  slight  amount  of  phosphorus,  not  sufficient  to  affect 
the  weight  materially,  was  always  found  in  the  deposit 
of  copper. 

2.  From  Antimony: — 

In  tartrate  solution.  In  the  presence  of  one-tenth 
of  a  gram  of  each  metal,  making  certain  that  the  anti- 
mony is  in  its  highest  state  of  oxidation,  add  8  grams 
of  tartaric  acid  and  30  c.c.  of  ammonia  (sp.  gr.  0.91). 
Electrolyze  at  50°  with  a  current  of  N.D100  =  o.o8- 
o.io  ampere  and  1.8-2  volts.  Total  dilution  150  c.c. 
The  ordinary  temperature.  Time,  5  hours  (J.  Am. 
Ch.  S.,  15,  195). 

Smith  and  Wallace  (Jr.  An.  Ch.,  7,  189;  Z.  f.  anorg. 
Ch.,  4,  274)  have  also  used  this  separation  with  emi- 
nent success.  They,  too,  emphasize  the  necessity  of 
having  the  antimony  in  its  highest  form  of  oxidation. 
Several  examples  will  illustrate  their  method  of  pro- 
cedure : — 


1 84 


ELECTRO-ANALYSIS. 


Z 

_^ 

i       ».' 

"  £  a 

JjH 

o 

off 

S  2  w 

P 

S 

H 

S  K 

3 

o 

O 

o  o 

sis 

CL, 

1° 

Q 

><i 

° 

Ufa 

0.0670 

0.1449 

175  c.c. 

15  c.c. 

3-4 

1.8 

O.I 

0.0670 

0.1341 

0.1449 

175    " 

15  " 

3-4 

2.0 

O.  I 

0.1341 

0.1341 

0.2898 

175    " 

15  " 

3-4 

2.0 

0.08 

0.1344 

The  deposited  metal  showed  no  antimony. 
See  also  Puschin  and  Trechzinsky,  Ch.  Z.,  28,  482; 
also  Elektrochemische   Zeitschrift,    14,   47. 

From  Arsenic : — 

(a)  In  ammoniacal  solution.  McCay  (Ch.  Z.,  14,  509) 
observed  that  a  current  conducted  through  a  potas- 
sium arsenate  solution,  made  distinctly  ammoniacal, 
had  no  effect  upon  the  arsenic,  while  with  copper  under 
like  conditions  the  metal  was  quantitatively  precipi- 
tated. Upon  this  behavior  he  has  based  a  very  excel- 
lent separation  of  the  two  metals.  Care  should  be 
taken  not  to  introduce  too  much  ammonia  water.  In 
this  laboratory  the  method  of  McCay,  with  the  condi- 
tions here  presented,  has  repeatedly  given  excellent 
results : — 

Add  20  c.c.  of  ammonium  hydroxide  (sp.  gr.  0.91) 
and  2.5  grams  of  ammonium  nitrate  to  the  solution 
containing  0.2121  gram  of  copper  and  0.1540  gram  of 
arsenic;  dilute  to  125  c.c.  with  water,  heat  to  5o°-6o°, 
and  electrolyze  with  N.D100  =  o.5  ampere  and  3.5 
volts.  The  copper,  precipitated  in  three  hours,  weighed 
0.2123  and  0.2121  gram.  Drossbach  (Ch.  Z.,  16,  819) 
and  Oettel  confirm  (Ch.  Z.  (1890),  14,  509)  (also  see 
Copper)  McCay's  experience. 

Freudenberg,  who  adopted  the  suggestion  of  Kili- 


SEPARATION    OF    METALS COPPER.  185 

ani,  of  giving  more  attention  to  the  pressure  than  to 
the  amperage,  succeeded  in  separating  copper  and 
arsenic  (latter  existing  as  arsenate)  by  arranging  to 
have  in  their  solution,  30  c.c.  in  excess  of  a  10  per 
cent,  ammonium  hydroxide  solution  and  then  elec- 
trolyzing  with  a  current  of  1.9  volts  until  the  liquid 
became  colorless,  which  usually  occurred  after  from 
6-8  hours  (Z.  f.  ph.  Ch.,  12,  118). 

With  a  rotating  anode  (p.  72).  Dilute  the  solution 
to  125  c.c.,  add  25  c.c.  of  ammonium  hydroxide  (sp. 
gr.  0.74),  and  2.5  grams  of  ammonium  nitrate,  then 
electrolyze  with  N.D100  =  5  amperes  and  7  volts. 
Fifteen  minutes  will  suffice  to  precipitate  0.2742  gram 
of  copper  from  an  equal  amount  of  arsenic.  The  de- 
posit will  be  smooth  and  adherent  (J.  Am.  Ch.  S., 
26,  1285). 

Schmucker  separated  copper  from  arsenic  with  con- 
ditions similar  to  those  indicated  for  copper  and  anti- 
mony in  ammoniacal  tartrate  solution  (see  above). 

(b)  In    potassium    cyanide   solution.     Add    the    copper 
solution  to  that  of  the  alkaline  arsenite  or  arsenate,  and 
then  introduce  a  solution  of  potassium  cyanide  until  the 
precipitate  first  produced  is  just  dissolved;  the  liquid 
will  then  show  a  slight  purple  tint.     Electrolyze  with 
the  following  conditions:  N.D100  =  0.25-0.26  ampere; 
volts  =  2. 4-3. 6;    dilution,    150    c.c.;    time,    3    hours; 
temperature,  60°. 

(c)  In  acid  solution.     Freudenberg  adds   10—20  c.c.  of 
dilute  sulphuric  acid  to  the  solution  of  the  metals  in 
question  and  then  electrolyzes  with  a  current  having  a 
tension  of   1.9  volts.     The  arsenic  existed  partly  as 
trioxide  and  partly  as   pentoxide.     The  precipitation 
was  made  during  the  night  (Z.  f.  ph.  Ch.,  12,  117). 

17" 


1 86  ELECTRO-ANALYSIS. 

Copper  present,  0.3000  gram;  found,  0.2997  gram; 
arsenic  present,  0.3531  gram.  The  copper  was  always 
brilliant  in  color. 

The  separation  can  also  be  made  in  nitric  acid  solu- 
tion with  the  same  voltage.  It  is  inferior  to  the  first 
method. 

By  using  the  rotating  anode  and  following  the  con- 
ditions recommended  in  the  separation  of  copper  from 
aluminium  by  the  same  procedure  (p.  182)  excellent 
results  may  be  obtained  (J.  Am.  Ch.  S.,  26,  1285). 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  and 
the  Alkali  Metals.     The  conditions  given  for  the  sepa- 
ration of  copper  from  aluminium  in  nitric  acid  solution 
(p.  181)  will  serve  for  its  separation  from  these  metals. 

5.  From  Bismuth.     See  the  separation  of  bismuth  from 
copper,  p.  227. 

6.  From  Cadmium: 

(a)  In  nitric  acid  solution.  It  was  in  a  solution  contain- 
ing free  nitric  acid  that  these  two  metals  were  first 
separated  electrolytically  (Am.  Ch.  Jr.,  2,  41).  The 
results  have  been  frequently  confirmed.  An  idea  of 
the  proper  working  conditions  may  be  obtained  from 
the  following:  To  a  solution  in  which  were  present 
0.0988  gram  of  copper  and  0.1152  gram  of  cadmium 
were  added  2  c.c.  of  nitric  acid  of  sp.  gr.  1.43.  The 
total  dilution  of  the  liquid  equaled  100  c.c.  It  was 
heated  to  50°  and  electrolyzed  with  N.D100  =  o.io 
ampere  and  2.5  volts.  In  3  hours  the  copper  was 
completely  precipitated.  It  was  bright  in  color  and 
weighed  0.0988  gram.  It  contained  no  cadmium  (J. 
Am.  Ch.  S.,  19,  873;  also  Jr.  An.  Ch.,  7,  253). 

When   the   copper   has   been   precipitated,   washed, 


SEPARATION    OF    METALS COPPER.  187 

dried,  and  weighed,  make  the  residual  liquid  alkaline 
with  sodium  hydroxide,  add  sufficient  potassium  cy- 
anide to  redissolve  the  precipitate,  and  electrolyze  as 
directed  on  p.  81. 

This  separation  may  be  performed  in  a  few  minutes 
with  the  rotating  anode  by  following  the  conditions  pre- 
scribed under  the  separation  of  copper  from  aluminium 
(p.  182)  in  the  same  electrolyte  (J.  Am.  Ch.  S.,  26, 

1285). 

(b)  In  sulphuric  acid  solution.  From  solutions  in 
which  there  is  free  sulphuric  acid  the  copper  may  be 
electrolytically  precipitated,  leaving  the  cadmium. 
This  is  evidenced  by  the  following  examples :  Total 
dilution,  100  c.c. ;  10  c.c.  of  sulphuric  acid,  sp.  gr. 
1.09;  0.1975  gram  of  copper  and  0.1828  gram  of  cad- 
mium; N.D100  —  0.05-0.07  ampere  and  1.70-1.76 
volts;  at  the  ordinary  temperature.  The  precipitate 
of  copper  weighed  0.1976  gram  (Am.  Ch.  Jr.,  12, 
no).  By  heating  the  electrolyte  the  time  can  be  re- 
duced to  8  hours. 

The  separation  has  also  been  made  by  strict  atten- 
tion to  difference  in  potential  (Freudenberg,  Z.  f.  ph. 
Ch.,  12,  116).  Ten  to  twenty  cubic  centimeters  of 
dilute  sulphuric  acid  are  added  to  the  solution  con- 
taining the  two  metals  and  the  liquid  is  then  electro- 
lyzed  with  a  current  not  exceeding  2  volts.  The  cop- 
per will  be  deposited  very  rapidly  and  be  free  from 
cadmium. 


COPPER  TAKEN. 
0.2734  gram 
0.4101   gram 
0.3000  gram 

CADMIUM  TAKEN. 
0.2560  gram 
0.2958  gram 
0.4437  gram 

COPPER  FOUND. 
0.2729  gram 
0.4098  gram 
0.3003  gram 

These   separations  were  conducted   during  the  night. 


1 8  8  ELECTRO-ANALYSIS. 

Heidenreich  (Ber.,  29,  1585)  met  with  success  in  ap- 
plying Freudenberg's  suggestion,  but  asserts  that  the 
tension  should  not  exceed  1.8  volts  for  N.D100  = 
0.07-0.05  ampere.  See  also  Denso,  Z.  f.  Elektrochem., 
9,  469. 

(c)  In  phosphoric  acid  solution.  The  separation  of  the 
two  metals  in  the  presence  of  free  phosphoric  acid  has 
often  been  made  in  this  laboratory  with  satisfaction. 
Favorable  conditions  will  be  found  in  the  example 
which  appears  here:  Dilution  of  solution,  125  c.c. ; 
0.2452  gram  of  metallic  copper  and  0.1827  gram  of 
metallic  cadmium;  20  c.c.  of  disodium  hydrogen  phos- 
phate, sp.  gr.  1.0353,  and  10  c.c.  of  phosphoric  acid, 
sp.  gr.  1.347;  temperature,  60°;  N.D100  =  0.07-0.08 
ampere  and  2.5  volts;  time,  3  hours  (Am.  Ch.  Jr.,  12, 

329)- 

7.  From   Calcium.     See   the   separation   of   copper   from 

barium,  p.  186. 

8.  From  Chromium.     See  copper  from  aluminium,  p.  182, 

for  the  conditions  of  separation  when  the  metals  are 
present  in  nitric  or  sulphuric  acid  solution.  This  state- 
ment also  holds  true  if  the  rotating  anode  be  used  in  the 
same  electrolytes  (J.  Am.  Ch.  S.,  26,  1285). 
(a)  In  phosphoric  acid  solution.  Volume  of  solution 
(containing  0.1239  gram  of  metallic  copper  and  0.1403 
gram  of  metallic  chromium  as  sulphates)  225  c.c.,  60 
c.c.  of  disodium  hydrogen  phosphate  (sp.  gr.  1.033) 
and  8  c.c.  of  phosphoric  acid  (sp.  gr.  1.347)  ;  N.D100  = 
0.062  ampere  and  2.5  volts;  temperature,  65°;  time,  6 
hours  (J.  Am.  Ch.  S.,  21,  1003). 

When  using  the  rotating  anode  follow  the  instruc- 
tions laid  down  for  the  separation  of  copper  from 
aluminium  in  this  electrolyte  (p.  .183)  (J.  Am.  Ch. 


SEPARATION    OF    METALS COPPER.  189 

S.,  26,  1285).     The  copper  will  contain  traces  of  phos- 
phorus. 

From  Cobalt:— 

(a)  In  the  presence  of  nitric  or  sulphuric  acid  the  sepa- 
ration of  these  two  metals  may  be  accomplished  by  ob- 
serving- the  conditions  given  for  the  separation  of  cop- 
per from  aluminium  in  the  presence  of  the  same  acids 
(see  p.   182).     Dr.  Wolcott  Gibbs  employed  mineral 
acid  solutions  for  this  purpose  many  years  ago  (Z.  f.  a. 
Ch.,  3,  334).     Most  analysts  prefer  the  sulphate  solu- 
tion.    Neumann  is  of  this  number.     He  dissolves,  for 
example,   i  gram  each  of  copper  sulphate  and  cobalt 
sulphate  in  the  requisite  volume  of  water,  adds  3  c.c.  of 
concentrated  sulphuric  acid,   dilutes  to    150  c.c.,  and 
electrolyzes  with  N.D100  =  i  ampere  at  the  ordinary 
temperature.     The  time  required  for  the  complete  pre- 
cipitation of  the  copper  varies  from  2^-3  hours.     The 
filtrate  or  solution  poured  off  from  the  deposit  of  cop- 
per need  only  be  mixed  with  an  excess  of  ammonia 
water  and  then  be  exposed  to  a  stronger  current  in 
order  to  precipitate  the  cobalt.     See  Z.  f.  angw.  Ch., 
17,  892. 

(b)  In  oxalic  acid  solution.     The  double  oxalates  have 
also  been  used.     The  method  requires  a  strict  adher- 
ence to  the  prescribed  voltage    (1.1—1.3)    to  yield  a 
satisfactory  result.     Classen,  with  whom  the  method 
originated,  advises  the  addition  of  6  grams  of  am- 
monium oxalate  to  the  solution  of  the  salts  and  acid- 
ulates   the    liquid    with    oxalic    acid,    acetic    acid,    or 
tartaric  acid.     Four  hours  are  required  for  the  pre- 
cipitation of  0.25  gram  of  copper  (Z.  f.  Elektrochem., 
i,    291,    292;    Ber.,    27,    2060).     Also    Puschin   and 
Trechzinsky,  Z.  f.  angw.  Chemie,  19,  892. 


ELECTRO-ANALYSIS. 

(c)  In  phosphoric  acid  solution.  An  example  will 
afford  an  idea  of  the  method  of  procedure :  Total 
dilution,  225  c.c. ;  60  c.c.  of  sodium  hydrogen  phos- 
phate (sp.  gr.  1.033)  ;  10  c.c.  of  phosphoric  acid  (sp. 
gr.  1.347);  N.D100  =  0.035  ampere  and  1.5  volts; 
temperature,  62° ;  time,  6  hours.  Copper  present, 
0.1239  gram;  cobalt  present,  o.iooo  gram.  Copper 
found,  0.1243  gram  (J.  Am.  Ch.  S.,  21,  1003;  Am. 
Ch.  Jr.,  12,  329;  Jr.  An.  Ch.,  5,  133). 

In  using  the  rotating  anode  to  bring  about  the  sepa- 
ration of  copper  from  cobalt  an  electrolyte  containing 
sulphuric  or  phosphoric  acid  should  not  be  employed. 
In  a  nitric  acid  electrolyte  the  separation  is  all  that 
can  be  desired.  Use  the  conditions  described  in  the 
separation  of  copper  from  aluminium  (p.  182)  (J.  Am. 
Ch.  S.,  26,  1286). 

10.  From  Gold.     See  p.  247. 

11.  From  Iron: — 

(a)  In  nitric  acid  solution.     The  conditions  given   for 
the  separation  of  copper  from  aluminium  (p.  182)  will 
answer  here.     When  much   iron   is   present,   difficul- 
ties will  be  encountered.     The  copper  tends  to  redis- 
solve  (Schweder,  Berg-Hutt.  Z.,  36,  5,  n,  31). 

(b)  In  sulphuric  acid  solution.     Experience  has   dem- 
onstrated that  the  separation  of  the  metals  in  ques- 
tion is  best  and  most  accurately  made  in  the  presence 
of   free   sulphuric  acid,   observing  the   conditions   as 
described  on  p.  182  for  copper  from  aluminium.     When 
the  copper  has  been  fully  precipitated,  which  usually 
requires  2j  hours,  the  residual  solution  is  poured  off, 
the  copper   is   washed,   and   the   liquid   reduced   to   a 
suitable  volume,  neutralized  with  ammonia,  and  4-6 


SEPARATION    OF    METALS COPPER.  IQI 

grams  of  ammonium  oxalate  introduced  into  the 
liquid,  which  is  then  electrolyzed  at  3O°-4O°  with  a 
current  of  N.D100  —  1-1.5  amperes  and  3.4-3.8  volts. 
The  iron  will  be  fully  precipitated  in  3-4  hours  (Clas- 
sen, Neumann). 

(c)  In  phosphoric  acid  solution.     In  this  laboratory  suc- 
cess has  attended  the  use  of  the  phosphates   in  the 
presence  of  free  phosphoric  acid.     Recently  the  proper 
conditions    as    to    current    density   and    voltage   have 
been  carefully  determined.     It  will  be  seen  from  the 
appended  example  that  the  results  are  most  satisfac- 
tory :  Total  dilution,  225  c.c. ;  disodium  hydrogen  phos- 
phate, 60  c.c.   (sp.  gr.  1.0358)  ;  10  c.c.  of  phosphoric 
acid  (sp.  gr.   1.347);  temperature,  53°  C. ;  N.D100  = 
0.04  ampere  and  2.4  volts;   time,   7  hours.     Copper 
present,  0.1239  gram;  found,  0.1237  gram  (Am.  Ch. 
Jr.,  12,  329;  Jr.  An.  Ch.,  5,  133;  J.  Am.  Ch.  S.,  21, 
1002). 

The  use  of  the  rotating  anode  may  be  resorted  to 
in  each  of  the  preceding  electrolytes  with  most  satis- 
factory results,  if  the  conditions  mentioned  on  p.  182 
for  the  separation  of  copper  from  aluminium  be  care- 
fully observed  (J.  Am.  Ch.  S.,  26,  1286). 

(d)  In  animoniacal  solution.     In  such  a  solution  Vort- 
mann  separates  the  copper  from  a  large  quantity  of 
iron.     The  liquid  containing  the  two  metals  is  mixed 
with  ammonium  sulphate  and  an  excess  of  ammonia 
water.     The  author  maintains  that  the  ferric  hydrox- 
ide, which  is  of  course  precipitated,  does  not  interfere 
with  the  deposition  of  the  copper.     The  latter  is  free 
from  iron.     The  current  employed  in  this  separation 
should  be  N.D100  —  0.1-0.6  ampere   (M.   f.  Ch.,   14, 
552). 


1 92  ELECTRO-ANALYSIS. 

It  is  doubtful  whether  the  copper  is  really  free 
from  iron.  The  opinion  presented  under  the  separa- 
tion of  nickel  from  iron  (p.  264)  and  the  experiences 
there  recorded  certainly  make  this  recommendation 
very  questionable.  Indeed,  in  this  laboratory  it  was 
found  in  'separating  the  copper  from  iron  in  chalco- 
pyrite  by  this  method  that  if  the  precipitation  of  the 
former  took  place  in  a  platinum  dish  it  was  invariably 
contaminated  with  iron.  On  the  other  hand,  if  the 
solution  of  metals  was  placed  in  a  beaker  and  a 
vertical  platinum  plate  was  made  the  cathode,  then 
the  copper  deposited  was  free  from  iron.  The  ferric 
hydrate  floating  about  in  the  platinum  dish  and  in  im- 
mediate contact  with  the  precipitate  is  partially  reduced 
to  the  metallic  form. 

(e)  In  oxalic  acid  solution.  This  procedure  is  due  to 
Classen  (Ber.,  27,  2060),  who  adds  to  the  solution 
containing  both  metals  in  the  form  of  sulphates  from 
6-8  grams  of  ammonium  oxalate  and  sufficient  oxalic, 
acetic,  or  tartaric  acid  to  render  the  liquid  acid.  The 
total  dilution  is  150  c.c.  N.D100=  i  ampere;  voltage, 
2.9-3.4  at  50°-6o°.  Time,  3  hours.  It  is  absolutely 
necessary  to  replace  the  oxalic  acid  as  it  is  decomposed, 
otherwise  iron  will  separate  upon  the  copper.  The 
method  requires  the  strictest  attention  to  details,  other- 
wise its  results  will  be  far  from  satisfactory.  Indeed, 
its  omission  from  the  last  edition  of  Classen's  "  Quanti- 
tative Electrolysis  "  would  seem  to  indicate  that  its 
author  had  lost  faith  in  its  efficacy. 

(/)  To  a  solution  of  copper  sulphate  and  pure  ferrous 
sulphate  add  1.5  gram  of  pure  potassium  cyanide  and 
10  c.c.  of  ammonia  (sp.  gr.  0.94),  then  dilute  to  100 
c.c.,  rotate  the  anode  about  400  revolutions  per  minute 


SEPARATION    OF    METALS COPPER.  1 93 

and  electrolyze  with  a  current  of  N.D100  =  9  to  n 
amperes  and  10  volts.  The  copper  will  be  fully  pre- 
cipitated, free  from  iron,  in  ten  minutes  (J.  Am.  Ch. 
S.,  29,  455). 

12.  From  Lead.  The  separation  of  these  two  metals  has 
great  value  from  the  technical  standpoint.  It  is  fortu- 
nate, therefore,  while  both  separate  under  the  influence 
of  the  current  in  a  nitric  acid  solution,  that  they  are 
deposited  at  opposite  poles.  Very  considerable  atten- 
tion has  been  paid  to  the  conditions  which  ought  to  pre- 
vail during  the  deposition.  Many  writers  have  con- 
tributed their  experience  on  this  point,  and  from  them 
is  gathered  the  following:  The  liquid  electrolyzed  should 
equal  150  c.c.  in  volume.  It  should  contain  15  c.c.  of 
nitric  acid  and  be  heated  to  about  60°  and  acted  upon 
with  a  current  of  N.D100  —  1-1.5  amperes  and  1.4  volts. 
In  the  course  of  an  hour  all  the  lead  will  have  been  pre- 
cipitated upon  the  anode, — which  in  this  separation  should 
be  a  dish  with  roughened  surface, — but  not  all  of  the 
copper  will  have  been  deposited  on  the  cathode — a  smaller, 
perforated  dish.  It  will  be  noticed  in  the  course  of  the 
decomposition  that  the  lead  separates  first  and  the  copper 
more  slowly.  When  the  lead  is  fully  precipitated,  wash 
without  interrupting  the  current,  proceed  further  as  di- 
rected on  p.  101,  and  after  placing  the  liquid  and  wash 
water,  reduced  to  130  c.c.,  into  another  weighed  dish, 
make  the  latter  the  cathode  and  suspend  in  it  the  smaller 
dish  upon  which  some  copper  had  been  deposited,  making 
it  the  anode.  The  solution  will  give  up  its  copper  on 
passing  the  current  and  the  metal  will  be  deposited  on  the 
larger  vessel  (the  cathode).  It  may  be  well  to  add  that 
the  liquid  poured  from  off  the  lead  dioxide  will  be  quite 
18 


1 94  ELECTRO-ANALYSIS. 

acid,  therefore  neutralize  it  with  ammonium  hydroxide 
and  add  10  c.c.  of  nitric  acid.  The  electrolysis  can  then 
be  conducted  with  N.D100  =  i  ampere  and  2.2-2.5  volts, 
at  the  ordinary  temperature. 

13.  From  Magnesium.     See  the  separation  of  copper  from 
barium,  etc.,  p.  186. 

Copper  may  be  separated  from  magnesium  in  an  elec- 
trolyte containing  nitric,  sulphuric  or  phosphoric  acid, 
with  the  help  of  the  rotating  anode,  by  observing  the 
conditions  given  under  the  separation  of  copper  from 
aluminium,  pp.  182,  183  (see  J.  Am.  Ch.  S.,  26,  1286). 

14.  From  Manganese: — 

(a)  In  sulphuric  acid  solution.     It   should  be  remem- 
bered that  from  such  a  solution  the  manganese  will 
be  deposited  upon  the  anode  as  peroxide  (see  p.  134)  ; 
therefore,  in  the  electrolysis  let  the  larger  dish,  with 
rough  inner  surface,  be  made  the  anode  to  receive 
the    manganese.     The    solution    containing    the    two 
metals   is   diluted  to    130-150  c.c.   with  the  addition 
of  10  drops  of  concentrated  sulphuric  acid.     Let  the 
current  be  N.D100  =  0.5-1.0  ampere.    The  most  favor- 
able temperature  is   5o°-6o°.     The  time  required  is 
usually  2-3  hours.     Experience  has  taught  that  too 
much  manganese  must  not  be  present.     When  the  de- 
position is  finished,  treat  the  deposit  as  already  des- 
cribed on  p.  135.     The  washing  should  be  performed 
without  interrupting  the  current. 

(b)  In  nitric  acid  solution.     The  separation  can  also  be 
effected  in  the  presence  of   free  nitric  acid.     If  the 
content  of   the  latter,   however,   exceeds   3   to  4  per 
cent.,   instead   of  having  the  manganese   precipitated 
on  the  anode  it  remains  in  solution  and  a  red  color 


SEPARATION  OF  METALS — COPPER.  195 

appears  at  the  anode  due  to  permanganic  acid.  In 
the  actual  analysis,  the  solution  of  the  two  metals 
ought  to  be  acidulated  with  a  few  cubic  centimeters 
of  acid  and  then  electrolyzed  at  60°  with  the  same 
current  conditions  as  given  in  a. 

It  will  be  wise  here  to  observe  the  statement  made 
upon  page  135  as  to  the  influence  of  the  strong  min- 
eral acids.  Indeed,  if  this  be  true,  then  the  preced- 
ing separations  are  worthless  and  should  be  discarded, 
as  has  been  done  with  the  separation  in  oxalate  so- 
lutions. In  the  writer's  personal  experience  the  sepa- 
ration in  sulphuric  acid  solution  does  give  satisfac- 
tory results.  The  subject  deserves  further  investi- 
gation. 

The  rotating  anode  may  be  used  in  both  a  sulphuric 
or  nitric  acid  electrolyte  to  effect  this  separation  if  the 
conditions  under  copper  from  aluminium  (p.  182)  are 
observed  (J.  Am.  Ch.  S.,  26,  1287). 
(c)  In  phosphoric  acid  solution.  When  free  phosphoric 
acid  is  present  in  the  solution  containing  salts  of  these 
metals,  no  question  need  arise  as  to  the  result,  for 
oft-repeated  tests,  made  in  this  laboratory,  have  amply 
demonstrated  the  accuracy  of  the  procedure.  The 
appended  example  will  illustrate:  N.D100  =  o.o5  am- 
pere; voltage  =2. 5;  temperature,  56°;  time,  6  hours; 
dilution,  225  c.c. ;  copper  present,  0.1239  gram;  copper 
found,  0.1236  gram;  manganese  present,  0.1200  gram: 
60  c.c.  of  disodium  hydrogen  phosphate  (sp.  gr. 
1.038)  ;  10  c.c.  of  phosphoric  acid  (sp.  gr.  1.347)  (J. 
Am.  Ch.  S.,  21,  1004,  and  Am.  Ch.  Jr.,  12,  329). 

The  copper  deposit  in  this,  as  well  as  in  the  many 
other  trials  conducted  under  practically  the  same  con- 
ditions, was  deep  red  in  color  and  very  adherent.  It 


196  ELECTRO-ANALYSIS. 

contained  no  manganese.  The  latter  does  not  even 
appear  at  the  anode,  except  as  an  amethyst  color,  indi- 
cating the  formation  there  of  permanganic  acid. 

15.  From  Mercury.     See  the  separation  of  mercury  from 
copper,  pp.  218,  219. 

1 6.  From  Molybdenum.     Add  1.5  grams  of  pure  potas- 
sium cyanide  to  the  solution  of  the  two  metals ;  dilute 
with  water  to  150  c.c.,  heat  to  60°,  and  electrolyze  with 
N.D100  =  o.28   ampere   and   4   volts.     The   copper   will 
be  completely  precipitated  in  5-6  hours. 

17.  From  Nickel: — 

(a)  In  acid  solution.     This  separation  may  be  realized 
by  observing  the  conditions  given  for  the  separation 
of  copper  from  aluminium   (p.    182)    or  those  noted 
under  copper  from  cobalt  (p.  189).     That  is,  in  nitric 
or  sulphuric  acid  solution  (Wolcott  Gibbs,  Z.  f.  a.  Ch., 
3,  334),  the  separation  is  all  that  the  analyst  can  ask. 
The  separation  in  oxalate  solution,  as  recommended 
by  Classen  (Z.  f.  Elektrochem.,  i,  291,  292),  must  also 
be  executed  with  conditions  analogous  to  those  indi- 
cated for  copper  from  cobalt,  b    (p.    189).     Also  Z. 
f.  Elektrochem.,  9,  469. 

(b)  In  phosphoric  acid  solution.     The  writer  has  found 
that  in  the  presence  of  free  phosphoric  acid  this  separa- 
tion  can  be   made  with   ease  and   the  confidence   of 
securing  a   favorable   result:   copper   present,   0.1239 
gram;    copper    found,    0.1241    gram;    nickel    present, 
0.1366  gram;  60  c.c.  of  disodium  hydrogen  phosphate, 
sp.  gr.  1.033;  IO  c-c-  of  phosphoric  acid,  sp.  gr.  1.347; 
total  dilution,  225  c.c.;  N.D100  =  0.035  ampere;  ten- 
sion =  1.5  volts;  time,  6  hours;  temperature,  62°  C. 
(J.  Am.  Ch.  S.,  21,  1003).     For  the  conditions  when 


SEPARATION    OF    METALS COPPER.  197 

iron,  cobalt,  zinc,  and  copper  are  present  together  in 
phosphoric  acid  solution,  see  J.  Am.  Ch.  S.,  21,  1004. 

In  attempting  to  separate  these  two  metals  in  a  sul- 
phuric or  phosphoric  acid  electrolyte,  using  a  rotating 
anode,  the  results  were  poor,  but  in  an  electrolyte  con- 
taining nitric  acid,  they  were  most  satisfactory. 

To  the  solution  containing  0.2500  gram  of  each 
metal  add  0.25  cubic  centimeter  of  concentrated  nitric 
acid  and  three  grams  of  ammonium  nitrate.  Elec- 
trolyze  with  a  current  of  N.D100  =  4  amperes  and  a 
pressure  of  5  volts.  In  fifteen  minutes  the  separa- 
tion will  be  complete.  The  speed  of  rotation  of  the 
anode  should  be  about  600  revolutions  per  minute. 

To  show  how  helpful  this  separation  may  be  an 
analysis  of  a  nickel  coin  will  be  here  given : 

Dissolve  the  coin  (4.925  grams  in  weight)  in  20 
cubic  centimeters  of  concentrated  nitric  acid  diluted 
with  an  equal  volume  of  water.  Exactly  neutralize 
with  ammonium  hydroxide,  transfer  to  a  250  cubic 
centimeter  measuring  flask  and  fill  this  to  the  mark 
with  water.  Transfer  25  cubic  centimeters  of  this 
liquid  to  a  weighed  platinum  dish,  and  add  three  grams 
of  ammonium  sulphate,  then  dilute  with  water  to  125 
cubic  centimeters,  heat  almost  to  boiling  and  electro- 
lyze  with  a  current  of  N.D100  =  5  amperes  and  a 
pressure  of  5.5  volts  for  twenty  minutes.  (The  pre- 
cipitated copper  in  this  particular  analysis  weighed 
0.3691  gram  =  74.95  per  cent,  of  the  coin.)  Pre- 
cipitate the  nickel  from  the  solution  with  sodium  hy- 
droxide and  bromine  water,  filter  and  wash.  Dissolve 
the  precipitate  in  2  cubic  centimeters  of  concentrated 
sulphuric  acid  diluted  with  water,  add  30  cubic  centi- 
meters of  concentrated  ammonium  hydroxide,  dilute  to 


198  ELECTRO-ANALYSIS. 

125  cubic  centimeters,  heat  and  electrolyze  with  a  cur- 
rent of  N.D100  =  6  amperes  and  a  pressure  of  5 
volts.  (In  twenty  minutes  0.1217  gram,  correspond- 
ing to  24.71  per  cent,  of  nickel,  was  precipitated.)  The 
solution  from  the  nickel  deposit  should  be  filtered  to 
get  the  iron — in  this  particular  case  it  weighed  0.0026 
gram,  equivalent  to  0.35  per  cent,  of  metallic  iron. 

Two  and  one-half  hours  will  suffice  for  the  complete 
analysis  (J.  Am.  Ch.  S.,  25,  906). 

1 8.  From  Palladium.     See  the  following  separation: 

19.  From  Platinum.     Add  1.5  grams  of  pure  potassium 
cyanide  and   5   grams  of  ammonium  carbonate  to  the 
solution  of  the  two  metals,  dilute  with  water  to  125  c.c., 
heat  to  70°,  and  electrolyze  with  N.D100  =  o.2  ampere 
and   2-2.5    volts.     The   copper   will   be   precipitated   in 
6  hours. 

In  using  the  rotating  anode  add  to  the  solution  of  the 
two  metals,  3  grams  of  potassium  cyanide  and  10  to  20 
c.c.  of  ammonia.  Electrolyze  with  a  current  of  N.D100  = 
3  amperes  and  5  volts. 

20.  From  Potassium.     See  copper  from  barium,  etc.   (p. 
186). 

21.  From  Selenium. 

(a)  In   cyanide   solution.     To   the   solution   containing 
0.0745  gram  of  copper  and  0.2500  gram  of  sodium 
selenate  add  i  gram  of  potassium  cyanide,  dilute  to 
150  c.c.,  heat  to  60°  C.,  and  electrolyze  with  N.D100  = 
0.2   ampere   and  4  volts.     The  precipitation   will  be 
finished  in  five  hours. 

(b)  In  nitric  acid  solution.     To  a  solution  containing 
the  quantities  of  metal  as  in  (a)  add  I  c.c.  of  nitric 
acid  (sp.  gr.  1.43),  dilute  to  150  c.c,  and  electrolyze  at 


SEPARATION    OF    METALS COPPER.  1 99 

65°  C,  with  a  current  of  N.D100  =  0.05  to  0.08  am- 
pere and  2  to  2.5  volts. 

(c)  In  sulphuric  acid  solution.  Add  one  cubic  centi- 
meter of  concentrated  sulphuric  acid  to  the  solution 
of  the  metals  and  electrolyze  with  N.D100=o.O5  to 
o.io  ampere  and  2.25  volts  at  65°  C.  The  separa- 
tion will  be  complete  in  five  hours. 

22.  From  Sodium.     See  copper  from  barium,  p.  186. 

23.  From  Strontium.     See  copper  from  barium,  p.  186. 

24.  From  Silver.     See  silver  from  copper,  p.  240.     Classen 
proposed  to  precipitate  the  two  metals  with  ammonium 
oxalate,   silver  oxalate  being  insoluble  in  an  excess  of 
the  precipitant,  while  the  copper  salt  was  soluble.     The 
former   was   to   be   filtered   off,   dissolved   in   potassium 
cyanide,   and   electrolyzed,   while   the  filtrate   containing 
the  copper  was  to  be  subjected  to  a  separate  electrolysis. 
This  is  really  not  an  electrolytic  separation,  as  was  shown 
by  others  (J.  Am.  Ch.  S.,  16,  420).     Further,  the  copper 
deposits  were  invariably  found  to  contain  silver,  so  that 
it  is  best  not  to  follow  this  procedure. 

25.  From  Tellurium: — 

(a)  In  nitric  acid  solution.     For  several  years,  at  inter- 
vals, experiments  have  been  made  in  this  laboratory  by 
D.  L.  Wallace,  upon  the  electrolytic  separation  of  these 
metals.     The  results  have  been  uniformly  good  with 
the  following  conditions:  Copper,   in  grams,  0.1543; 
tellurium,  in  grams,  o.uoi ;  dilution,  100  c.c. ;  0.5  c.c. 
nitric  acid  (sp.  gr.  1.42)  ;  N.D100  =  o.io  ampere  and 
2.06  volts ;  temperature,  66°-7O°  ;  time,  5  hours.    Cop- 
per found:  (a)   0.1541  gram;   (b)  0.1546  gram;  (c) 
0.1543  gram;   (d)  0.1542  gram. 

(b)  In  sulphuric  acid  solution.     Add  one  cubic  centi- 


200  ELECTRO-ANALYSIS. 

meter  of  concentrated  sulphuric  acid  to  the  solution  of 
the  metals,  dilute  to  150  c.c.,  heat  to  65°  C,  and  elec- 
trolyze  with  N.D100  =  0.05  to  o.i  ampere  and  2  to  2.25 
volts.  Six  hours  will  suffice  for  the  precipitation  of 
the  copper  (J.  Am.  Ch.  S.,  25,  895). 

26.  From  Thallium.     No  attempt  has  been  made  to  effect 
this  separation. 

27.  From   Tin.     Schmucker   demonstrated    (J.    Am.    Ch. 
S.,    15,    195)    that,  having  tin   in   its   highest   oxidation 
form,  it  is  possible  to  precipitate  and  separate  copper  from 
it  by  adding  to  the  solution  8  grams  of  tartaric  acid  and 
30  c.c.  of  ammonia  water  (sp.  gr.  0.91),  then  electrolyz- 
ing  at  50°  C.  with  N.D100  =  o.O4  ampere  and  1.8  volts. 
If  a  tenth  of  a  gram  of  each  metal  be  present,  the  copper 
will  be  precipitated  in  5  hours.     The  total  dilution  was 
175  c.c. 

As  observed  in  preceding  paragraphs,  this  method  was 
utilized  by  Schmucker  in  the  separation  of  copper  from 
arsenic  and  copper  from  antimony.  The  same  author 
also  separated  copper  from  a  mixture  of  antimony, 
arsenic,  and  tin,  using  the  conditions  as  described  above. 

Or,  when  antimony,  arsenic,  and  tin  are  associated 
with  copper,  treat  the  four  sulphides  with  sodium  sul- 
phide. The  resulting  alkaline  sulphide  solution  can  then 
be  employed  for  the  separation  of  the  first  three  (p.  251), 
while  the  insoluble  copper  sulphide  may  be  dissolved  and 
treated  as  described  on  p.  70. 

28.  From  Tungsten.     The  conditions  given  for  the  sepa- 
ration of  copper  from  molybdenum  (p.  196)  may  be  used 
for  this  separation. 

29.  From  Uranium:— 

(a)   In  nitric  acid  solution.     Add  0.5  c.c.  of  concentrated 


SEPARATION    OF    METALS COPPER.  2OI 

nitric  acid  to  the  solution,  dilute  to  150  c.c.,  heat  to 
60°,  and  electrolyze  with  N.D100  =  0.14-0.27  ampere 
and  2-2.4  volts.  The  copper  will  be  precipitated  in 
3  hours. 

(b)  In  sulphuric  acid  solution.  The  solution  of  these 
metals  should  be  mixed  with  2  c.c.  of  concentrated  sul- 
phuric acid,  diluted  to  150  c.c.  with  water,  heated  to 
50°-6o°,  and  electrolyzed  with  N.D100  =  0.16  ampere 
and  2  volts.  The  precipitation  will  be  complete  in  4 
hours. 

The  separation  of  copper  from  uranium  may  be 
readily  carried  out  with  the  help  of  a  rotating  anode  by 
observing  the  conditions  given  for  the  separation  of 
copper  from  aluminium  in  the  same  electrolytes  (p. 
182)  (J.  Am.  Ch.  S.,  26,  1287). 

30.  From  Vanadium.     A  method  of  separation  is  lacking. 

31.  From  Zinc: — 

(a)  In  nitric  acid  solution.  The  conditions  mentioned 
under  a  in  copper  from  aluminium  (p.  181),  and  under 
copper  from  cobalt  (p.  189)  and  nickel  (p.  196),  will 
answer  here  in  getting  a  satisfactory  separation.  The 
solution  must  be  kept  acid  during  the  decomposition. 
To  this  may  be  added,  that  to  a  solution  containing 
0.1341  gram  of  copper  and  equal  amounts  of  zinc, 
cobalt,  and  nickel,  5  c.c.  of  nitric  acid  were  added,  the 
liquid  was  diluted  to  200  c.c.,  and  electrolyzed  with 
0.04  ampere,  when  0.1339  gram  of  copper  was  obtained. 
In  using  the  rotating  anode  in  conducting  this  sepa- 
ration add  to  the  solution  of  the  metals  3  grams  of 
ammonium  nitrate  and  0.25  c.c.  of  concentrated  nitric 
acid,  then  electrolyze  with  a  current  of  N.D100  =  5 
amperes  and  9  volts.  Time,  15  minutes. 


202  ELECTRO-ANALYSIS. 

(b)  In   sulphuric   acid   solution.     The     conditions     are 
analogous   to  those   employed   for   the   separation   of 
copper  from  aluminium  (p.  182),  cobalt  (p.  189),  and 
nickel  (p.  196). 

In  this  electrolyte  also  the  separation  is  greatly 
accelerated  by  the  use  of  the  rotating  anode.  Dilute 
the  solution  to  125  c.c.,  add  i  c.c.  of  sulphuric  acid  of 
sp.  gravity  1.83  and  electrolyze  with  N.D100  =  3  to  5 
amperes  and  5  volts.  Time,  10  minutes. 

(c)  In  oxalate  solution.     This  method  (Ber.,  17,  2467) 
is   no   longer   recommended.     Only   the   most   careful 
observance  of  the  conditions  given  will  yield  anything 
like  a  satisfactory  result. 

(d)  In  phosphoric  acid  solution  (Am.  Ch.  Jr.,  12,  329; 
Jr.  An.  Ch.,  5,  133).     The  early  suggestions  that  these 
metals  be  precipitated  as  phosphates  and  the  latter  be 
then  dissolved  in  phosphoric  acid  and  the  resulting  solu- 
tion   be    electrolyzed    were    not    favorably    received. 
Here,  in  this  laboratory,  where  the  separation  had  been 
repeatedly   performed,   the  method   gave   satisfaction. 
To  extend  its  application  the  most  favorable  conditions 
have  been  worked  out  and  repeated.     They  are  given 
in  the  example  which  follows : 

To  the  solution  of  the  sulphates,  containing  0.1239 
gram  of  copper  and  a  like  quantity  of  zinc,  were  added 
60  c.c.  of  disodium  hydrogen  phosphate  (sp.  gr.  1.033) 
and  10  c.c.  of  phosphoric  acid  (sp.  gr.  1.347).  It  was 
diluted  to  225  c.c.,  heated  to  60°,  and  electrolyzed  with 
N.D100  =  0.035  ampere  and  2.5  volts,  for  5  hours, 
when  0.1244  gram  of  copper  was  obtained,  free  from 
zinc. 

By  following  the  conditions  given  in  the  separation 
of  copper  from  aluminium  (p.  183)  in  this  electrolyte 


SEPARATION    OF    METALS CADMIUM.  203 

a  rotating  anode  will  prove  most  helpful.  Traces  of 
phosphorus  will  appear  in  the  copper  deposits. 

Another  interesting  separation,  properly  belonging 
here,  was  that  of  copper  from  a  mixture  of  iron,  cobalt, 
and  zinc.  The  solution  diluted  to  225  c.c.  contained : — 

0.1239  gram  of  copper 

0.1007  gram  of  cobalt 

o.i  ooo  gram   of  iron 

0.1200   gram   of  zinc 

30  c.c.  of  Na,HPO4  (sp.  gr.  1.0358) 

15  c.c.   of  H3PO4  (sp.  gr.   1.347) 

It  was  electrolyzed  at  57°  with  a  current  of  N.D100  = 
0.04-0.05  ampere  and  2.3  volts.  In  six  hours  the 
copper  was  fully  precipitated.  It  weighed  0.1240  gram 
and  contained  none  of  the  other  metals  (J.  Am.  Ch.  S., 
21,  1003,  1004). 

CADMIUM. 

The  ordinary  gravimetric  methods  for  the  determination 
of  this  metal  are  such  that  they  can  frequently  with  advan- 
tage be  replaced  by  the  electrolytic  process.  The  same  is 
true  when  it  comes  to  the  separation  of  cadmium  from  the 
metals  usually  associated  with  it,  as  well  as  those  with  which 
it  occasionally  occurs.  The  writer  prefers  the  electro- 
lytic course  whenever  it  is  available.  To  what  extent  the 
various  suggestions  offered  for  the  electrolytic  determination 
of  the  metal  can  be  applied  in  separations  may  be  gathered 
from  the  following  paragraphs  : — 

i.  From  Aluminium:— 

(a)  In  sulphuric  acid  solution.  In  this  separation  it  is 
only  necessary  to  add  to  the  solution  of  the  salts  of  the 
metals  3  c.c.  of  sulphuric  acid,  of  specific  gravity  1.09, 


2O4  ELECTRO-ANALYSIS. 

dilute  to  125  c.c.  with  water,  heat  to  65°,  and  electro- 
lyze  with  N.D100  —  0.078  ampere  and  2.61  volts. 
The  cadmium  will  be  deposited  in  the  course  of  from 
4-4/2  hours.  It  should  be  washed  without  interrupt- 
ing the  current.  In  one  case  o.  1 1 1 1  gram  of  Cd  in- 
stead of  0.1105  was  found;  in  another,  0.1181  instead 
of  o.i 1 88  gram;  and  in  a  third,  0.1604  instead  of 
0.1599  gram. 

To  demonstrate  the  advantage  in  using  a  rotating 
anode  in  making  this  separation  an  example  in  actual 
experimentation  may  be  here  introduced : 

To  a  solution  containing  0.2727  gram  of  cadmium 
and  0.2500  gram  of  aluminium  add  I  c.c.  of  sulphuric 
acid  (sp.  gr.  1.83),  dilute  to  125  c.c.  with  water  and 
electrolyze  with  a  current  of  N.D100  =  5  amperes  and 
5  volts.  Time  ten  minutes.  The  deposits  are  per- 
fectly adherent  (J.  Am.  Ch.  S.,  26,  1288).  Or,  by 
using  a  mercury  cathode  and  rotating  anode  with  a 
current  of  3  amperes  and  7  volts,  total  volume  of  the 
solution  being  10  c.c.,  this  separation  may  be  made  in 
twenty  minutes. 

(b)  In  phosphoric  acid  solution.  Add  an  excess  of  di- 
soclium  hydrogen  phosphate  (sp.  gr.  1.0358)  to  the 
solution  of  the  metals  and  then  sufficient  phosphoric 
acid  (sp.  gr.  1.347)  to  leave  about  1.5  c.c.  of  the  latter 
in  excess.  Dilute  with  water  to  100  c.c.,  heat  to  50°, 
and  electrolyze  with  N.D100  =  0.06  ampere  and  3  volts. 
Time,  7  hours.  See  p.  82  for  further  details  (J.  Am. 
Ch.  S.,  20,  279;  Am.  Ch.  Jr.,  12,  329;  13,  206). 

When  using  the  rotating  anode  dilute  the  solution 
of  the  metal  salts  to  125  c.c.  after  adding  10  c.c.  of 
phosphoric  acid,  and  50  c.c.  of  a  10  per  cent,  solution 
of  disodium  hydrogen  phosphate  solution  and  elec- 


SEPARATION    OF    METALS CADMIUM.  2O5 

trolyze  with  a  current  of  N.D100  =  5  amperes  and  7 
volts  for  10  minutes  (J.  Am.  Ch.  S.,  26,  1288). 

2.  From  Antimony.     Schmucker  (J.  Am.  Ch.  S.,  15,  195) 
used  for  this  purpose  the  method  described  on  p.    183 
for  the  separation  of  copper  from  antimony,  observing 
the  same  conditions.     The  results  were  perfectly  satis- 
factory.    In  washing  the  cadmium  deposit  water  alone 
was  used.     The  deposition  was  made  during  the  night, 
but  by  heating  the  electrolyte  the  time   factor  can  be 
much  reduced. 

3.  From  Arsenic: — 

(a)  In  animoniacal  tartrate  solution.     Proceed  precisely 
as  directed  on  p.  184  in  the  separation  of  copper  from 
arsenic  (J.  Am.  Ch.  S.,  15,  195). 

(b)  In  alkaline  cyanide  solution.     After  converting  the 
arsenic  into  its  highest  state  of  oxidation,  add  from 
2  to  3  grams  of  potassium  cyanide  to  the  solution  con- 
taining the  metals  and  electrolyze  with  a  pressure  not 
exceeding  2.6  volts   (Am.  Ch.  Jr.,  12,  428;  Z.  f.  ph. 
Ch.,  12,  122). 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  and 
the  Alkali  Metals.     No  records  of  any  such  separations 
have  been  made. 

5.  From  Beryllium.     There  is  no  record  of  this  separation. 

6.  From  Bismuth.     See  separation  of  bismuth  from  cad- 
mium, p.  225. 

7.  From  Chromium.     The  conditions  given  for  the  sepa- 
ration of  cadmium  from  aluminium  will  answer  equally 
well  in  this  case;  also  when  applying  a  rotating  anode 
in  a  phosphoric  acid  electrolyte  (J.  Am.  Ch.  S.,  26,  1288). 

In   the   presence   of    3   cubic   centimeters    of   concen- 
trated  sulphuric   acid,   using  the   mercury   cathode  and 


2O6  ELECTRO-ANALYSIS. 

rotating  anode,  this  separation  is  easily  made  with  a 
current  of  2  to  3  amperes  and  3.5  to  4  volts.  Time  25 
minutes. 

8.  From  Cobalt:— 

(a)  In  sulphuric  acid  solution.     Use  the  conditions  pre- 
scribed for  the  separation  of  cadmium  from  aluminium 
(p.  204).     It  may  be  well  to  add  that  the  addition  of 
ammonium  sulphate  to  the  solution  is  advantageous. 
The  voltage  should  not  exceed  2.8-2.9. 

(b)  In  alkaline  cyanide  solution.     Add  4-5   grams  of 
pure  potassium  cyanide  to  the  solution  of  the  metals, 
dilute  to  200  c.c.,  and  electrolyze  with  N.D100  =  o.3 
ampere  and  2.6  volts  (Am.  Ch.  Jr.,  12,  104;  Z.  f.  ph. 
Ch.,  12,  116).     See  also  J.  Am.  Ch.  S.,  27,  1286. 

9.  From  Copper.     See  also  copper  from  cadmium,  pp.  186, 

187,  1 88.  In  addition  to  the  methods  used  in  separat- 
ing these  metals,  in  which  the  copper  is  precipitated,  we 
may  add  the  following :  Introduce  5  to  6  grams  of  pure 
potassium  cyanide  into  the  solution  of  the  metals  for 
every  0.2-0.4  gram  of  cadmium  and  copper.  Dilute 
the  solution  to  200  c.c.  and  electrolyze  with  a  current 
of  N.D100  =  0.02-0.04  ampere  and  2.6-2.7  volts.  The 
cadmium  will  be  deposited;  the  copper  will  remain 
dissolved  (Jr.  An.  Ch.,  3,  385;  Z.  f.  ph.  Ch.,  12,  122). 
Rimbach  (Z.  f.  a.  Ch.,  37,  288)  has  tried  this  separa- 
tion with  marked  success  in  the  analysis  of  aluminium- 
cadmium-tin  alloys  containing  copper  as  impurity.  In 
case  the  nitrate  of  cadmium  is  used  it  will  be  necessary 
to  increase  the  current  to  N.D100  =  0.4  ampere. 

10.  From  Gold.     This  separation  is  not  recorded.     It  is 
probable  that  it  can  be  executed  in  a  hot  alkaline  cy- 
anide solution. 


SEPARATION    OF    METALS CADMIUM.  2O/ 

IT.  From  Iron: — 

(a)  In  sulphuric  acid  solution.'    Follow  the  directions 
given  in  a  under  cadmium  from  aluminium,  p.  204. 
It  may  be  observed  that  this  is  the  procedure  used, 
too,  in  separating  cadmium  from  chromium.     See  the 
separation  of  cadmium  from  aluminium   (p.  204)   for 
the  conditions   to  be  used  when  applying  a  rotating 
anode  (J.  Am.  Ch.  S.,  26,  1288). 

(b)  In  phosphoric  acid  solution.     Again  the  conditions 
noticed  in  b  under  cadmium  from  aluminium  (p.  204) 
will  prove  to  be  very  satisfactory  in  this  particular 
case  (J.  Am.  Ch.  S.,  26,  1289). 

(c)  In  potassium  cyanide  solution.     Dissolve  a  mixture 
of  cadmium  and  ferrous  sulphates  in  100  c.c.  of  water, 
previously  acidulated  with  a  few  drops  of  dilute  sul- 
phuric acid,  introduce  2  to  3  grams  of  pure  potassium 
cyanide,  and  heat  gently  until  perfect  solution  ensues. 
If  considerable  time  elapses  before  the  liquid  becomes 
yellow  in  color,  add  a  few  drops  of  caustic  potash. 
Dilute  the  liquid  to  200  c.c.  and  electrolyze  the  cold 
solution  with  a  current  of  N.D100  =  0.05-0.1  ampere. 
The  deposit  of  cadmium  will  be  very  satisfactory  (W. 
Stortenbeker,  Z.  f.  Elektrochem.,  4,  409). 

It  is  possible,  by  using  the  rotating  anode,  to  per- 
form this  separation  in  twenty  minutes  by  electrolyz- 
ing  the  solution  of  mixed  salts,  after  the  addition  of 
12  grams  of  potassium  cyanide  and  2  grams  of  sodium 
hydroxide,  with  a  current  of  N.D100  =  5  amperes 
and  a  pressure  of  5  volts.  It  is  well  to  use  a  quarter 
of  a  gram  of  each  metal  (J.  Am.  Ch.  S.,  27,  1285). 

12.  From  Lead.     See  lead  from  cadmium,  p.  234. 

13.  From  Magnesium.     See  cadmium  from  barium,  etc., 
p.  205.     In  this  connection  it  may  be  stated  that  Rim- 


208  ELECTRO-ANALYSIS. 

bach  (Z.  f.  a.  Ch.,  37,  289)  effected  this  separation  in  a 
potassium  cyanide  solution.  The  precaution  is  made 
that  not  too  much  magnesia  be  present,  ammonium 
chloride  also  being  added  to  the  solution  to  hold  up  the 
magnesia.  The  current  strength  best  adapted  for  this 
separation  proved  to  be  N.D100  =  0.02-0.05  ampere. 
The  time  was  14  hours. 

In  a  formic  acid  solution.  To  the  solution  of  the 
salts  of  the  two  metals  add  0.2  gram  of  sodium  carbon- 
ate and  12  c.c.  of  formic  acid  of  sp.  gr.  1.06,  then  elec- 
trolyze  with  a  current  of  N.D100— 5  amperes  and  6 
volts.  The  anode  should  perform  about  600  revolu- 
tions per  minute.  Ten  minutes  will  answer  for  the  full 
precipitation  of  the  cadmium  (J.  Am.  Ch.  S.,  27,  1285). 

In  electrolytes  of  sulphuric  and  phosphoric  acid  the 
conditions  applicable  here  are  found  under  cadmium  from 
aluminium,  p.  204. 
14.  From  Manganese: — 

(a)  In  sulphuric  acid   solution.     As   manganese    sepa- 
rates readily  from  a  sulphate  solution  in  the  presence 
of  a  slight  excess  of  sulphuric  acid,  and  then,  too, 
upon  the  anode  (p.  134),  it  is  only  necessary  to  add 
from  2  to  3  c.c.  of  sulphuric  acid  (sp.  gr.  1.09)  to  the 
solution  of  the  metals,  dilute  to  125  c.c.,  and  electro- 
lyze  with  the  current  and  voltage  given  under  cad- 
mium from  aluminium,  a.     As  the  manganese  is  pre- 
cipitated upon  the  anode  as  dioxide,  make  the  larger 
dish  the  receiving  vessel  for  it;  further,  let  its  inner 
surface   be    roughened.     The    cadmium    is    deposited 
upon  the  cathode.     The  method  has  been  used  in  this 
laboratory  with  success. 

(b)  In  phosphoric  acid  solution.     An  idea  of  the  ac- 
curacy of  the  method  can  be  best  obtained  from  an 


SEPARATION    OF    METALS CADMIUM.  2OQ 

actual  example.  The  conditions  also  for  work  will  be 
most  satisfactorily  learned  from  it.  Twenty  cubic 
centimeters  of  disodium  hydrogen  phosphate  (sp.  gr. 
1.0358)  and  3  c.c.  of  phosphoric  acid  (sp.  gr.  1.347) 
were  added  to  a  solution  containing  0.2399  gram  of 
cadmium  and  o.iooo  gram  of  manganese  and  the 
liquid  then  diluted  with  water  to  150  c.c.  and  electro- 
lyzed  at  the  ordinary  temperature  with  a  current  of 
i  ampere.  In  12  hours  0.2394  gram  of  cadmium  was 
precipitated.  There  was  not  the  slightest  deposition 
of  manganese  at  the  anode.  The  cadmium  deposit 
was  crystalline  in  appearance.  It  was  washed  with 
hot  water.  Before  the  final  interruption,  the  cur- 
rent ought  to  be  increased  and  allowed  to  act  for  an 
hour.  The  acid  liquid  should  be  removed  with  a 
siphon  before  disconnecting  (Am.  Ch.  Jr.,  13,  206). 

In  using  the  rotating  anode  as  an  aid  in  this  sepa- 
ration, according  to  (a)  and  (b)  follow  the  condi- 
tions given  under  the  separation  of  cadmium  from 
aluminium,  p.  204  (J.  Am.  Ch.  S.,  26,  1289). 

15.  From  Mercury.     See  mercury  from  cadmium,  p.  217. 

1 6.  From  Molybdenum.     The  alkaline  cyanide  solution 

is  well  adapted  for  this  purpose.  Add  from  1.5  to  3 
grams  of  pure  potassium  cyanide,  dilute  to  200  c.c.,  and 
electrolyze  at  40°  C,  with  N.D100  =  0.03-0.04  ampere 
and  2.25-3.0  volts.  The  conditions  are  practically 
those  used  in  the  separation  of  cadmium  from  arsenic 
(Am.  Ch.  Jr.,  12,  428). 

17.  From  Nickel: — 

(a)   In  sulphuric  acid  solution.     To  the  solution  of  salts 
of  the  two  metals  add  2  to  3  c.c.  of  sulphuric  acid,  sp. 
19 


2 1 0  ELECTRO-ANALYSIS. 

gr.  1.09,  also  ammonium  sulphate,  and  electrolyze 
with  the  current  density  and  voltage  mentioned  in 
the  separation  of  cadmium  from  aluminium,  a,  p.  204. 
The  conditions  favorable  to  the  use  of  the  rotating 
anode  in  this  separation  are  analogous  to  those  out- 
lined under  the  separation  of  cadmium  from  alu- 
minium, p.  204. 

(b)  In  phosphoric  acid  solution.     0.1827  gram  of  cad- 
mium and  0.1500  gram  of  nickel  (both  as  sulphates) 
were  precipitated   by   40   c.c.   of   disodium   hydrogen 
phosphate,  dissolved  in  3  c.c.  of  phosphoric  acid  (sp. 
gr.  1.347),  diluted  to  125  c.c.,  and  electrolyzed  at  the 
ordinary    temperature    with    N.D100  =  0.035    ampere 
and  2.5-3.0  volts.     The  precipitated  cadmium  weighed 
0.1820  gram.     It  was  washed  and  treated  as  directed 
upon  p.  81. 

(c)  In  alkaline  cyanide  solution.     The  solution  contain- 
ing the   double  cyanides   of   the  two  metals   is   well 
suited  for  this  separation,  but  it  is  absolutely  neces- 
sary to  have  a  little  free  sodium  hydroxide  present. 
The  conditions  would  be  then  about  as  follows :  Add 
to  the  solution  containing  0.1723  gram  of  cadmium, 
and  0.1600  gram  of  nickel,  2  grams  of  potassium  or 
sodium  hydroxide  and  3  grams  of  potassium  cyanide. 
Dilute  to  175  c.c.  and  electrolyze  at  40°  with  N.D100  —-- 
0.03-0.04  ampere -and  2.25-3.0  volts   (Am.   Ch.  Jr., 
12,  104;  Freudenberg,  Z.  f.  ph.  Ch.,  12,  122). 

1 8.  From  Osmium.  The  only  recorded  separation  of 
these  two  metals  was  made  in  a  solution  of  potassium 
cyanide.  The  quantity  of  cyanide  was  1.5  grams  for 
0.3  gram  of  the  combined  metals.  The  dilution  of  the 
solution  equaled  170  c.c.;  it  was  electrolyzed  with  a 


SEPARATION    OF    METALS  -  CADMIUM.  211 

current  of  N.D100  =  o.26  ampere  and  3-4  volts.     Time, 
10  hours;  temperature,  25°   (Jr.  An.  Ch.,  6,  87). 

An   electrolytic   separation   of   cadmium    from   plati- 
num and  palladium  is  not  known  (Am.  Ch.  Jr.,  12,  428; 


ig.  From  Selenium.     This  separation  has  not  been  made. 

20.  From  Silver.     See  p.  239,  for  silver  from  cadmium. 

21.  From  Sodium.     See  the  separation  of  cadmium  from 
barium,  etc.,  p.  205. 

22.  From  Srontium.     See  the  separation  of  cadmium  from 
barium,  etc.,  p.  205. 

23.  From    Tellurium.     There    is    no    known    electrolytic 
separation. 

24.  From  Tin.     They  have   not  been   separated   electro- 
lytically. 

25.  From  Tungsten.    The  conditions  detailed  in  the  sepa- 
ration  of  cadmium   from  arsenic    (p.   205)    and  under 
cadmium  from  molybdenum    (p.   209)   in  cyanide  solu- 
tion will  answer  here. 

26.  From  Uranium.     The  current  has  not  been  used  in 
their  separation. 

27.  From  Vanadium.     They  have  not  been  separated  in 
the  electrolytic  way. 

28.  From  Zinc.     As  these  two  metals  are  so  frequently 
found  together,  both  in  natural  and  in  artificial  prod- 
ucts, it  is  not  surprising  that  electrolytic  methods  have 
been  sought  to  effect  their  separation  in  such  a  manner 
as  to  leave  no  doubt  in  the  mind  of  the  analyst.     They 
should   be   and   indeed    are   preferable   to   the   ordinary 
gravimetric  procedures. 


2  I  2  ELECTRO-ANALYSIS. 

The  first  method  proposed  and  published  was  that  by 
Yver  (B.  s.  Ch.  Paris,  34,  1 8).  It  is  based  upon  the 
fact  that  cadmium  separates  well — 

(a)  In  acetate  solution.     Convert  the  metals  into  ace- 
tates   by  the  addition  of    2  to  3  grams  of    sodium 
acetate  to  their  solution,  followed  by  several  drops  of 
free  acetic   acid.     Dilute  the  liquid  to    100  c.c.   and 
warm    to    70°  C.      Electrolyze    with    N.D100=o.io 
ampere  and  2.2  volts.     Time,  3-4  hours.     The  cad- 
mium (0.2  gram)  will  be  precipitated  in  a  crystalline 
form  and  free  from  zinc  (Am.  Ch.  Jr.,  8,  210). 

The  zinc  in  the  liquid  from  the  cadmium  deposit 
may  then  be  precipitated  by  the  method  of  Riche 
(p.  114). 

Mention  may  be  here  made  of  the  fact  that  Smith 
and  Knerr  (Am.  Ch.  Jr.,  8,  210)  electrolyzed  a  solu- 
tion of  cadmium  and  zinc  to  which  3-4  grams  of 
sodium  tartrate  and  tartaric  acid  had  been  added, 
with  a  current  of  N.D100  =  0.3-0.4  ampere  and  2.25- 
3  volts.  The  temperature  of  the  solution  was  60°. 

(b)  In  oxalic  acid  solution.  Eliasberg  (Z.  f.  a.  Ch.,  24, 
55°)  proposed  this  method,  second  in  point  of  time, 
and  recommended  the  following  procedure:  Dissolve 
the   metallic   oxides    in   hydrochloric   acid,   evaporate 
their  solution  to  dryness,  take  up  the  residue  in  water, 
add    to    the    liquid    8    grams    of   potassium    oxalate 
(C2O4K2)     and    2     grams     of    ammonium     oxalate 
((NH4)2C2O4),  dilute  to  120  c.c.,  heat  to  8o°-85°, 
and  electrolyze  with  N.D100  =  0.01-0.02  ampere  and  3 
volts.     The  cadmium  will  be  precipitated   free   from 
zinc.     See  also  Waller,  Z.   f.   Elektrochem.,  4,  241- 
247.     From  6  to  7  hours  are  required  for  the  deposi- 
tion of  0.2  gram  of  cadmium. 


SEPARATION    OF    METALS CADMIUM.  213 

(c)  In  sulphuric  acid  solution.     To  the  liquid  containing 
the  salts  of  the  two  metals  add  3  to  4  c.c.  of  a  concen- 
trated ammonium  sulphate  solution  and  follow  with 
2  to  3  c.c.  of  dilute  sulphuric  acid.     Dilute  to  100  c.c. 
and  electrolyze  with  N.D100  =  0.08  ampere  and  2.8- 
2.9    volts     (Neumann's    Elektrolyse,    p.     189).     See 
Denso,  Z.  f.  Elektrochem.,  9,  469. 

In  the  electro-chemical  laboratory  of  the  Univer- 
sity of  Munich  the  separation  of  cadmium  from  zinc 
is  in  a  certain  sense  a  combination,  of  c  and  a.  For 
example,  sodium  hydroxide  is  added  to  the  sulphates 
of  the  metals  until  a  permanent  precipitate  is  formed; 
this  is  then  dissolved  in  as  little  sulphuric  acid  as  pos- 
sible, the  solution  is  diluted  to  70  c.c.  and  the  cad- 
mium precipitated  by  a  current  of  N.D100  =0.07  am- 
pere. When  the  greater  portion  of  this  metal  has 
been  thrown  out  of  the  solution,  the  free  sulphuric 
acid  is  neutralized  with  sodium  hydroxide  and  2  to  3 
grams  of  sodium  acetate  are  introduced  into  the 
liquid,  which  is  heated  to  45°  and  electrolyzed  with  a 
current  of  N.D100  =  0.03  ampere  and  3.6  volts. 

(d)  In  phosphoric   acid  solution.     Total   dilution,    125 
c.c. ;  cadmium,  0.1827  gram;  zinc,  0.1500  gram;  di- 
soclium  hydrogen  phosphate  (sp.  gr.   1.038),  40  c.c.; 
phosphoric  acid  (sp.  gr.  1.347),  3  c.c.;  N.D100  =  0.035 
ampere;  V=  2.5-3.0.    Cadmium  found,  0.1820  gram. 
The  ordinary  temperature.     Time,  10  hours  (Am.  Ch. 
Jr.,  12,  329). 

(e)  In    potassium    cyanide    solution.     This    separation 
originated  in  this  laboratory  (Am.  Ch.  Jr.,  n,  352). 
Example:    0.2426    gram    of    cadmium    as    sulphate, 
0.2000  gram  of  zinc  as  sulphate;  4.5  grams  of  po- 
tassium  cyanide;   total   dilution,    200   c.c.     Ordinary 


214  ELECTRO-ANALYSIS. 

temperature.      N.D100  =  0.03    ampere ;    volts  =  2.8- 
3.2.     0.2429  gram  of  cadmium  found. 

In  the  filtrate  the  zinc  may  be  precipitated  by  in- 
creasing the  current.  Freudenberg  used  this  method 
with  success,  applying  a  current  corresponding  to  an 
electromotive  force  of  2.6-2.7  volts. 

MERCURY. 

Experience  has  proved  that  this  metal  is  most  accu- 
rately determined,  and  most  satisfactorily  separated  from 
the  metals  usually  found  with  it  by  the  use  of  electrolytic 
methods  which  in  this  instance  are  preferable  in  every 
particular  to  the  ordinary  gravimetric  courses ;  hence  all  the 
known  separations  in  the  electrolytic  way  will  be  given,  in 
the  paragraphs  which  follow,  with  such  detail  that  no  doubt 
need  remain  as  to  the  final  results. 

While  mercury  is  very  quickly  determined  with  the  help 
of  the  rotating  anode  it  is  almost  impossible  to  separate  it 
from  other  metals,  owing  to  the  readiness  with  which  it 
forms  amalgams.  It  was,  however,  separated  in  a  beauti- 
ful mirror-like  form  from  aluminium  and  magnesium. 
i.  From  Aluminium:— 

(a)  In  nitric  acid  solution  (p.  181).     Add  3  c.c.  of  con- 
centrated nitric  acid  to  the  solution  of  the  two  salts, 
dilute  to  125  c.c. ;  heat  to  70°  C.,  and  electrolyze  with 
N.D100  =  0.06  ampere  and  2  volts.     Time,  2  hours. 
The  solution  in  the.  dish  must  be  siphoned  off  before 
the  interruption  of  the  current. 

(b)  In  sulphuric  acid  solution  (p.  182).     Add  i  c.c.  of 
sulphuric  acid  to  the  solution  of  the  salts;  dilute  to  125 
c.c.,  heat  to  65°  and  electrolyze  with  N.D100  =  0.4-0.6 


SEPARATION    OF    METALS MERCURY.  21  5 

ampere  and  3.50  volts.  The  mercury  (0.1500  gram) 
will  be  precipitated  in  an  hour.  Wash  it  with  cold 
water  and  proceed  as  directed  on  p.  92. 

From  Antimony.  Add  to  the  solution,  containing 
about  equal  amounts  of  the  two  metals,  5  grams  of  tar- 
taric  acid  and  15-20  c.c.  of  ammonia  water  (10  per 
cent.)  ;  dilute  to  175  c.c.,  and  electrolyze  with  N.D100  = 
0.015-0.085  ampere  and  2.2-3.5  volts.  The  temperature 
should  be  50°.  About  6  hours  will  be  required  for  the 
precipitation  (J.  Am.  Ch.  S.,  15,  205).  The  antimony 
must  exist  in  solution  as  an  antimonic  compound.  The 
method  was  first  worked  out  by  Schmucker  ( loc.  cit. )  and 
was  later  successfully  confirmed  by  Freudenberg  in  his 
study  of  the  differences  in  potential  (Z.  f.  ph.  Ch.,  12, 
112),  when  he  employed  an  electromotive  force  of  1.6-1.7 
volts.  Mercury  used,  0.2362  gram;  mercury  found, 
°-2356  gram;  antimony  present,  0.2600  gram. 

The  liquid  from  the  deposit  of  mercury,  after  acidula- 
tion,  may  be  precipitated  with  hydrogen  sulphide  and  the 
resulting  sulphide  be  dissolved  in  sodium  sulphide  and 
treated  as  described  on  p.  172  for  the  determination  of 
the  antimony. 

From  Arsenic: — 

(a)  In  nitric  acid  solution.     The  solution  of  the  metals 
should  contain  a  few  cubic  centimeters  of  free  nitric 
acid  and   then  be  acted   upon   with  an  electromotive 
force  of  1.7-1.8  volts:  Mercury  taken,  0.2380  gram; 
mercury  found,  0.2380  gram;  arsenic  present,  0.2516 
gram  (Freudenberg,  Z.  f.  ph.  Ch.,  12,  in). 

(b)  In  potassium   cyanide  solution.     Add   3   grams   of 
pure  potassium  cyanide  to  the  liquid  containing  0.5 
gram  of  combined  metals,  dilute  to  200  c.e.,  and  elec- 


2l6  ELECTRO-ANALYSIS. 

trolyze  with  N.D100  =  0.015  ampere  and  2.2-3.5  volts 
for  5  hours  at  65°  (Am.  Ch.  Jr.,  12,  428).  It  is  im- 
material whether  the  arsenic  is  present  as  an  arsenite  or 
arsenate. 

(c)  In  alkaline  sulphide  solution  (p.  92).  An  example 
will  best  illustrate  the  method :  To  the  solution  of  mer- 
cury add  25  c.c.  of  sodium  sulphide  (sp.  gr.  1.19), 
dilute  with  water  to  125  c.c.,  heat  to  70°  C.,  and  elec- 
trolyze  with  a  current  of  N.D100  =  o.n  ampere  and 
2.5  volts.  The  time  for  precipitation  is  usually  5  hours. 
See  Jr.  Fr.  Ins.,  1891. 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  and 
the  Alkali  Metals.     Use  method  a  under  mercury  from 
aluminium  (p.  214)  for  this  purpose. 

5.  From  Bismuth.     The  statements  with  reference  to  the 
separation  of  these  two  metals  are  contradictory.     The 
experiments  conducted  in  this  laboratory   (Jr.  An.  Ch., 
7,  252)  showed  that  the  metals  were  coprecipitated  from 
a  nitric  acid  solution,  as  .one  from  many  examples  will 
illustrate:  The  solution  contained  0.1132  gram  of  mer- 
cury  and   0.0716   gram   of  bismuth.     Ten   cubic   centi- 
meters of  nitric  acid  of  specific  gravity  1.2  were  added 
and  the  liquid  diluted  with  water  to  200  c.c.,  and  elec- 
trolyzed  with  a  current  of  N.D100  =  o.O4  ampere  and 
1.6  volts. 

The  precipitation  of  the  metals  was  complete,  but  the 
mercury  contained  bismuth.  This  was  one  of  eight  trials 
which  resulted  similarly.  They  were  made  to  disprove  a 
statement  which  had  appeared  repeatedly  in  three  editions 
of  Classen's  Quantitative  Analyse  durch  Elektrolyse  (p. 
147,  2d  ed.),  despite  the  fact  that  the  same  writer  had  de- 
clared previously  (Ber.,  19,  325)  :  "  Bismuth  cannot  be 


SEPARATION    OF    METALS MERCURY.  217 

separated  from  mercury  in  this  manner.  Both  metals 
are  precipitated  simultaneously  from  an  acid  solution. " 

After  this  study  had  been  made,  Freudenberg  (Z.  f. 
ph.  Ch.,  12,  in),  by  adherence  to  the  idea  of  the  differ- 
ences in  potential,  gave  results  which  would  indicate  a 
complete  separation ;  a  few  cubic  centimeters  of  nitric  acid, 
of  sp.  gr.  1.2,  and  2-4  grams  of  ammonium  nitrate  are 
added  to  the  nitrate  solution  of  the  two  metals  and  the 
electrolysis  conducted  with  a  potential  of  1.3  volt.  Mer- 
cury used,  0.2380  gram;  mercury  found,  0.2376  gram; 
bismuth  present,  0.2694  gram.  As  Neumann  (Elektro- 
lyse,  p.  181)  remarks,  the  possible  current  strength  is  ex- 
ceedingly low,  hence  a  long  time  is  required  for  the  pre- 
cipitation of  the  mercury. 

While  the  writer  has  never  tested  the  recommendation 
of  Freudenberg,  his  experience  gathered  from  numerous 
attempts  on  the  part  of  his  students  inclines  him  to  say 
that  the  procedure  is  worthy  of  further  study  at  least. 

6.  From  Cadmium: — 

(a)  In  acid  solution.     The  nitric  acid  and  sulphuric  acid 
solutions  lend  themselves  quite  well  to  this  separation. 
The  proper  conditions  for  the  obtainment  of  satisfac- 
tory results  are  given  in  the  section  on  mercury  from 
aluminium,  paragraphs  a  and  b  (p.  214). 

(b)  In  alkaline  cyanide  solution.     The  solution  contained 
0.1182  gram  of  mercury  and  0.2206  gram  of  cadmium. 
Two  and  one-half  grams  of  pure  potassium  cyanide 
were  added,  and  the  liquid  was  then  diluted  with  water 
to  125  c.c.,  heated  to  65°,  and  acted  upon  with  a  .cur- 
rent of  N.D100  =  o.oi8  ampere  and   1.7  volts.     The 
precipitation  was  complete  in  7  hours  at  the  ordinary 
temperature  (J.  Am.  Ch.  S.,  21,  919  also  17,  612). 

20 


2  I  8  ELECTRO-ANALYSIS. 

7.  From  Calcium.     See  the  separation  of  mercury  from 
barium  (p.  216). 

8.  From  Chromium.     The  methqds  recommended  for  the 
separation  of  mercury  from  aluminium,  p.  214,  will  an- 
swer for  this  particular  purpose. 

9.  From  Cobalt:— 

(a)  In  acid  solutions.     See  p.  214,  under  mercury  from 
aluminium. 

(b)  In   alkaline    cyanide   solution.     The    solution    con- 
tained 0.1216  gram  of  mercury  and  o.iooo  gram  of 
cobalt.     The  liquid  was  diluted  to  100  c.c. ;  2  grams 
of  potassium  cyanide  were  added  to  it  and  the  liquid, 
then  heated  to  65°,  was  electrolyzed  with  N.D100  = 
0.025-0.03   ampere   and   2.06-2.7  volts   for   5   hours. 
The  mercury  found  equaled  0.1213  gram  and  0.1217 
gram.     Too  much  potassium  cyanide  exercises  a  re- 
tarding influence  on  the  precipitation  of  the  mercury 
(J.  Am.  Ch.  S.,  21,  918;  Am.  Ch.  Jr.,  12,  104). 

10.  From  Copper:— 

(a)  In  nitric  acid  solution.     Freudenberg  (Z.  f.  ph.  Ch., 
12,    in),  with  attention  to  voltage  alone,   separates 
these  metals  as  follows  :  To  their  solution  (the  nitrates) 
add  several  cubic  centimeters  of  nitric  acid    (sp.   gr. 
1.2)   and  2  to  4  grams  of  ammonium  nitrate,  after 
which  electrolyze  with  a  current  having  a  pressure  of 
1.3    volts.     Mercury    present,    0.2380    gram;    copper 
present,  0.1356  gram;  mercury  found,  0.2377  gram; 
copper  found,  0.1358  gram.     The  separation  was  made 
during  the  night. 

(b)  In  alkaline  cyanide  solution.     It  was  in  a  solution  of 
the  double  cyanides  of  these  metals  that   they  were 
first  separated  successfully  in  the  electrolytic  way  (Am. 


SEPARATION    OF    METALS MERCURY.  2  19 

Ch.  Jr.,  ii,  264).  At  the  time  it  was  thought  that  the 
separation  could  not  be  regarded  as  yielding  trust- 
worthy results  when  the  copper  exceeded  20  per  cent., 
but  about  two  years  subsequently  it  was  shown  (Jr. 
An.  Ch.,  5,  489)  that  by  careful  adjustment  of  the  cur- 
rent strength  the  quantity  of  copper  could  not  only 
equal,  but  exceed,  that  of  the  mercury  almost  indefi- 
nitely (Spare  and  Smith,  J.  Am.  Ch.  S.,  23,  579). 
The  time,  however,  was  still  an  important  factor,  and 
it  was  not  reduced  by  Freudenberg,  who  electrolyzed 
the  double  cyanides  with  a  pressure  of  2.5  volts,  in  the 
presence  of  2  to  4  grams  of  potassium  cyanide  (Z.  f. 
ph.  Ch.,  12,  113).  The  reduction  of  this  factor  was 
made  in  1894  (J.  Am.  Ch.  S.,  16,  42)  by  gently  warm- 
ing the  electrolyte.  It  then  became  possible  to  fully 
precipitate  the  mercury  in  three  and  one-half  hours. 
Since  then  the  separation  has  been  repeatedly  made 
both  with  mercury  and  copper  (J.  Am.  Ch.  S.,  21, 
917),  and  with  mercury,  copper,  cadmium,  zinc,  and 
nickel  simultaneously  present.  The  following  condi- 
tions will  prove  satisfactory  for  this  separation :  Mer- 
cury present,  0.1216  gram;  copper  present,  equal 
amount;  total  dilution,  125  c.c. ;  potassium  cyanide, 
2-3  grams;  temperature,  65°  ;  time,  2^—3  hours.  Mer- 
cury found,  0.1215  -gram  (Revay,  Z.  f.  Elektrochem., 

4,  313). 

11.  From  Gold.     This  separation  has  not  been  made.     See 
Z.  f.  ph.  Ch.,  12,  113. 

12.  From  Iron:— 

(a)  In  nitric  acid  solution.     Use   the   conditions   indi- 
cated under  a,  mercury  from  aluminium  (p.  214). 

(b)  In  sulphuric  acid  solution.     See   b  under  mercury 
from  aluminium. 


22O  ELECTRO-ANALYSIS. 

(c)  In  alkaline  cyanide  solution.  Dissolve  ferrous  am- 
monium sulphate  in  water;  conduct  sulphur  dioxide 
through  it  to  reduce  any  ferric  salt  which  may  be 
present,  nearly  neutralize  the  excess  of  acid  with  sodium 
carbonate,  mix  with  the  solution  of  the  silver  salt,  and 
add  from  2.5  to  4  grams  of  potassium  cyanide  for  0.2- 
0.4  gram  of  the  combined  metals ;  then  electrolyze  with 
N.D100  =  0.02-0.05  ampere  and  2.5  volts,  with  a  tem- 
perature of  70°.  The  total  dilution  should  equal  125 
c.c.  Time,  3-4  hours  (J.  Am.  Ch.  S.,  21,  920). 

13.  From    Lead.     To   the    solution,    containing   the    two 
metals  add  from  25  to  30  c.c.  of  nitric  acid  (sp.  gr.  1.3), 
dilute  to  175  c.c.  with  water,  and  electrolyze  with  a  cur- 
rent of  N.D100  =  0.13  to  0.18  ampere  and  2  volts,  at  30° 
for  4  hours.     It  will,  of  course,  be  understood  that  the 
lead  is  deposited  as  dioxide  upon  the  anode  while  the 
mercury  is  simultaneously  precipitated  on  the  cathode. 
Use  a  dish  as  anode  (Smith  and  Moyer,  Jr.  An.  Ch.,  7, 
252;  Z.  f.  anorg.  Ch.,  4,  267;  Heidenreich,  Ber.,  29,  1585; 
Z.  f.  Elektrochem.,  3,  151). 

14.  From  Magnesium.     See   the   separation   of   mercury 
from  barium,  etc.,  p.  216. 

15.  From  Manganese : — 

(a)  In  nitric  acid  solution.     See  the  conditions  under 
which  manganese  is  precipitated  as  dioxide  (p.  134). 
The  mercury  separates  at  the  cathode. 

(b)  In  sulphuric  acid  solution.     The  conditions  which 
should  be  observed  in  depositing  manganese  from  a 
solution  containing  free  sulphuric  acid  will  answer  in 
this  particular  separation   (p.   134).     The  larger  dish 
must,  of  course,  be  made  the  anode.     The  quantities 
of  the  two  metals  must  not  be  too  large. 


SEPARATION    OF    METALS MERCURY.  221 

1 6.  From   Molybdenum.     The   separation   is   readily   ef- 
fected in  an  alkaline  cyanide  solution,  using  the  conditions 
prescribed  under  b  in  the  separation  of  mercury  from 
arsenic  (p.  215). 

17.  From  Nickel: — 

(a)  In  nitric  acid  solution.     Follow  the  conditions  given 
under  a  in  the  separation  of  mercury  from  aluminium, 
p.  214. 

(b)  In  sulphuric  acid  solution.     Reproduce  the  condi- 
tions of  b  in  the  separation  of  mercury  from  aluminium, 
p.  214. 

(c)  In  alkaline  cyanide  solution.     An  example  will  illus- 
trate :  Mercury  present,  0.1216  gram;  nickel  present, 
0.1500  gram;  potassium  cyanide,  2-2.5  grams;  total 
dilution,  125  c.c. ;  N.D100  =  o.O4  ampere;  volts  =  1.7- 
2.2;  temperature,  65°;  time,  4  hours.     The  mercury 
found  equaled  0.1213  gram  (J.  Am.  Ch.  S.,  21,  918; 
Am.  Ch.  Jr.,  12,  104). 

1 8.  From  Osmium.     Follow  the  directions  for  the  separa- 
tion of  mercury  from  arsenic  in  an  alkaline  cyanide  solu- 
tion, p.  215.     In  this  separation  the  quantity  of  alkaline 
cyanide   should  not  exceed   1.5   gram   for  0.2  gram  of 
metal  (Am.  Ch.  Jr.,  12,  428;  13,  417;  Jr.  An.  Ch.,  6,  87). 

19.  From  Palladium.     Let  the  conditions  be  the  same  as 
those  given  for  the  separation  of  mercury  from  platinum 
(see  below)   (Am.  Ch.  Jr.,  12,  428). 

20.  From  Platinum.     Example:  Mercury  present,  0.1373 
gram;  platinum  present,  o.iooo  gram;  total  dilution,  125 
c.c. ;   potassium   cyanide,   3   grams;   N.D100  =  0.04-0.05 
ampere;  V  =  2.i;  temperature,  65°-75°;  time,  4  hours. 
The  mercury  found  equaled  0.1372  gram  (Am.  Ch.  Jr., 
13,  417;  J.  Am.  Ch.  S.,  21,  920). 


222  ELECTRO-ANALYSIS. 

21.  From   Potassium.     See   mercury   from   barium,   etc., 
p.  216. 

22.  From  Selenium.     To  the  solution  of  the  two  metals, 
each  about  one  quarter  of  a  gram  in  amount,  add  one 
gram  of  potassium  cyanide,  dilute  to  150  c.c.  with  water, 
heat  to  60°  C.,  and  electrolyze  with  N.D100  =  0.03  am- 
pere and  a  pressure  of  3   volts.     The  precipitation  of 
the  mercury  will  be  complete  in  five  hours. 

In  a  nitric  acid  electrolyte  the  separation  is  conducted 
with  ease  by  observing  the  conditions  followed  in  the 
separation  of  silver  from  selenium,  p.  245. 

23.  From  Silver.     These  metals  cannot  be  separated  elec- 
trolytically  either  in  an  acid  or  alkaline  cyanide  solu- 
tion.    Classen  precipitates  them  together,  and  after  ascer- 
taining  their   combined   weight   expels   the  mercury   by 
ignition  and  weighs  the  residual  silver. 

24.  From  Sodium.     See  barium,  p.  216. 

25.  From   Strontium.     See  mercury   from  calcium,   etc., 
p.  218. 

26.  From  Tellurium.     In  a  cyanide  solution  the  separa- 
tion cannot  be  made.     Most  favorable  results  were  ob- 
tained in  a  nitric  acid  electrolyte.     An  example  will  illus- 
trate.    To  a  solution  containing  0.1272  gram  of  mer- 
cury and  0.2500  gram  of  sodium  tellurate,  three  cubic 
centimeters  of  nitric  acid    (sp.   gr.    1.43)    were  added. 
After  dilution  to   150  c.c.  with  water  it  was  heated  to 
60°  C.,   and   electrolyzed   with   a   current  of   N.D]00  = 
0.04  to  0.05  ampere  and  a  pressure  of  2  to  2.5  volts.     In 
five  hours  the  precipitation  was  finished  (J.  Am.  Ch.  S., 
25,  895). 

27.  From  Tin: — 

(a)   In  alkaline  sulphide  solution.     The  conditions  men- 


SEPARATION    OF    METALS MERCURY.  223 

tioned  under  mercury  (p.  92)  will  answer  perfectly 
for  this  separation  (Jr.  Fr.  Ins.,  1891).  To  change 
the  sodium  sulpho-salt  in  the  filtrate  into  ammonium 
sulphostannate  consult  p.  167. 

(b)  In  ammoniacal  tartrate  solution.  A  solution  of  the 
two  metals  was  made  by  adding  mercuric  chloride  to 
tartaric  acid,  followed  by  ammonia  water  and  then 
diluting  with  water.  This  solution  was  then  mixed 
with  the  tin  salt  solution  and  the  combined  liquids 
electrolyzed  with  a  current  showing  a  pressure  of  from 
1.6-1.7  volts.  (See  the  separation  of  mercury  from 
antimony  in  tartrate  solution,  p.  215;  also  J.  Am.  Ch. 
S.,  15,  p.  204.) 

It  may  be  of  interest  to  state  that  the  conditions 
given  for  the  separation  of  mercury  from  antimony 
(p.  215),  and  those  just  employed  above  for  the  sepa- 
ration of  mercury  from  tin  have  been  successfully 
applied  by  Schmucker  (J.  Am.  Ch.  S.,  15,  204)  for 
the  electrolytic  separation  of  mercury  from  a  solu- 
tion containing  arsenic,  antimony,  and  tin,  the  only 
change  being  in  the  addition  of  an  increased  amount 
of  tartaric  acid  and  ammonium  hydroxide.  Example : 
Mercury,  0.0933  gram;  arsenic,  0.1009  gram;  anti- 
mony, 0.1031  gram;  tin,  o.iooo  gram;  tartaric  acid, 
8  grams;  ammonium  hydroxide  30  c.c. ;  dilution,  175 
c.c. ;  N.D100=:o.O5  ampere;  volts  =  1.7.  The  pre- 
cipitation made  at  60°  was  complete  in  6  hours. 

28.  From   Tungsten.     Use   conditions   corresponding   to 
those   employed   in   the   separation   of  mercury   from 
arsenic  in  an  alkaline  cyanide  solution  (p.  215). 

29.  From   Uranium.     There   is   no   recorded   electrolytic 
separation  of  these  metals,  but  it  is  quite  probable  that 


2  24  ELECTRO-ANALYSIS. 

methods  a  and  b,  under  mercury  from  aluminium  (p. 
214),  would  be  applicable  in  this  case. 

30.  From  Vanadium.     They  have  not  been  separated  by 
the  current. 

31.  From  Zinc: — 

(a)  In  acid  solutions  (nitric  or  sulphuric)  the  conditions 
mentioned  under  a  and  b,  in  the  separation  of  mer- 
cury  from  aluminium,   will  prove  perfectly  satisfac- 
tory (p.  214). 

(b)  In  alkaline  cyanide  solution.     This  separation  has 
been  made  repeatedly  with  excellent  success,  so  that 
perhaps  an  actual  example  will  give  all  the  data  neces- 
sary to  guide  others  in  making  the  separation :  Mer- 
cury present,  0.1158  gram;  zinc  present,  o.iooo  gram; 
potassium  cyanide,  1.5  to  2  grams;  dilution,  125  c.c. ; 
N.D100  =  0.025-0.05    ampere;   V  =  2«5    to    3;   time, 
4  hours;  temperature,  60°.     Mercury  found,  0.1155 
gram   (J.  Am.  Ch.  S.,  21,  919;  Jr.  Fr.  Ins.,  1889). 

(c)  In    phosphoric    acid   solution.     An    example    from 
many  results  will  show  the  conditions  which  should 
be  pursued  in  conducting  the  separation  in  a  solution 
such  as  just  indicated :  25  c.c.   of  mercuric  chloride 
=  0.1159  gram  of  metal;  25  c.c.  of  zinc  sulphate  = 
o.ioio  gram  of  metal;  60  c.c.  of  disodium  hydrogen 
phosphate  (1.038  sp.  gr.)  ;  10  c.c.  of  phosphoric  acid 
(1.347  sp.  gr.)  ;  total  dilution,  175  c.c.;  temperature, 
60°;    N.D100  =  o.oi     ampere;    V=i-5;    time,    4-5 
hours.     Mercury  found,  0.1163  gram  (J.  Am.  Ch.  S., 

21,    I006). 


SEPARATION    OF    METALS BISMUTH.  225 

BISMUTH. 

The  separations  of  this  metal  from  other  metals  in  the 
electrolytic  way  are  not  numerous,  but  they  are,  notwith- 
standing*, of  decided  help  to  the  analyst,  and  therefore 
will  be  here  presented  in  such  detail  as  is  known. 

1.  From  Aluminium.     The  conditions  give.n  under  bis- 
muth for  its  determination  in  a  nitric   (p.  96)   or  sul- 
phuric acid   (p.  97)    solution  can  be  here  used   for  its 
separation    from    aluminium.     Its    precipitation    as    an 
amalgam  (p.  96)  is  well  adapted  for  this  purpose. 

2.  From  Antimony.     To  the  solution  containing  the  two 
metals  add  5  grams  of  tartaric  acid,   15  c.c.  of  ammo- 
nium hydroxide,  dilute  to  175  c.c.  with  water,  and  elec- 
trolyze  with  a  current  of  N.D100  =  0.022  ampere  and 
1.8  volts  at  50°  for  6  hours  (J.  Am.  Ch.  S.,  15,  203). 

3.  From  Arsenic.     The  course  just  outlined  for  the  sepa- 
ration of  bismuth   from  antimony  will  answer  in  this 
case    (J.   Am.   Ch.   S.,    15,   202).     Neumann    (Elektro- 
lyse,  p.   185)   states  that  the  two  metals,  if  in  sulphate 
solution,  can  be  separated  with  a  current  having  an  E. 
M.  F.  of  1.9  volts. 

4.  From  Barium.     The  conditions  for  the  precipitation  of 
bismuth  from  nitric  acid  solution  (p.  96)  will  answer  for 
this  separation. 

5.  From  Cadmium.     This  separation  may  be  conducted 
in  the  presence  of  free  nitric  acid  (p.  96),  by  the  amal- 
gam method   (p.   96),  or  in  a  sulphuric  acid  solution. 
If  using  the  last  electrolyte,   proceed  as   follows:   Dis- 
solve 0.1500  gram  of  cadmium  metal  in  2  c.c.  of  concen- 
trated sulphuric  acid  (sp.  gr.  1.84)  and  to  this  solution 
add  another  of  0.15  gram  of  bismuth  and  i  c.c.  of  con- 


226 


ELECTRO-ANALYSIS. 


centrated  nitric  acid,  i  gram  of  potassium  sulphate,  and 
dilute  with  water  to  150  c.c.,  heat  to  50°,  and  electro- 
lyze  with  a  current  of  N.D100  =  0.025  ampere  and  2 
volts.  Time,  8  hours.  The  bismuth  will  be  deposited 
in  a  bright,  metallic  form  (Kammerer). 

6.  From  Calcium.     The  conditions  given  on  pp.  96,  97 
for  the  determination  of  bismuth  may  be  relied  upon  in 
making  this  separation. 

7.  From  Chromium.     Use  a  nitric  acid  solution  (p.  96), 
or  adopt  the  method  given  in  the  following  paragraph : — 

To  a  solution  of  bismuth  containing  0.1500  gram  of 
metal  and  I  c.c.  of  nitric  acid  (sp.  gr.  1.42)  add  0.5  gram 
of  potassium  sulphate,  2  c.c.  of  sulphuric  acid  (sp.  gr. 
1.84),  and  a  quantity  of  chrome  alum  equivalent  to 
0.1500  gram  of  chromium.  Dilute  to  150  c.c.  with  water 
and  electrolyze  with  a  current  strength  of  N.D100  = 
0.025  ampere  and  2  volts,  the  temperature  being  main- 
tained at  50°  C.  After  8  hours  the  deposition  will  be 
complete  and  the  bismuth  will  be  free  from  chromium. 

RESULTS. 


a 

X    . 
H  55 

XQ 

g 

M 

H 

5   n 

5! 

O 

H 

H 

o 

0 

H 

h 

S  W 

S  « 

D  2; 
|§ 

1 

|j 

x  2 
H 

H 

s 

H 

a 

0 

H  a 
gg 

pqH 

03* 

X 

C/3 

Q        : 

s 

H 

LO"5" 

H 

Grm. 

Grm. 

Grm. 

Grm. 

r.c. 

C.c. 

Hours. 

oc. 

Amp. 

0.1434 

0.1430 

O.I5OO 

0-5 

2 

200 

9 

50 

003 

2 

Gauze. 

0.1434 

o.  1428 

0.1500 

0-5 

2 

150 

9 

50 

0.025 

2 

Basket. 

0.1434 

o.  1434 

0.1500 

0-5 

2 

200 

8^ 

50 

0.025 

2 

Gauze. 

0.1434 

0.1428 

0.1500 

0-5 

2 

150 

8/^ 

50 

O.O2 

2 

Basket. 

0.1434 

o.  1430 

O.I5OO 

0-5 

2 

8/4 

50 

O.O2 

2 

Spiral. 

0.1434 

0.1429 

0.1500 

0-5 

2 

IS0 

9 

50 

O.O25 

2 

The  chromium  salt  seems  to  exert  a  beneficial  influ- 
ence  on   the  character  of   the   deposit.     Much   of   the 


SEPARATION    OF    METALS BISMUTH.  227 

chromium,  during  the  electrolysis,  is  oxidized  to  chromic 
acid.  Especially  is  this  true  when  gauze  electrodes  are 
used  (Kammerer). 

8.  From  Cobalt.  Proceed  as  in  the  separation  from  alu- 
minium (p.  225),  or  from  chromium  (above). 

g.  From  Copper.  In  a  nitric  acid  solution  copper  and  bis- 
muth cannot  be  separated  electrolytically.  This  state- 
ment has  been  the  subject  of  considerable  controversy 
in  past  years  (Z.  f.  anorg.  Ch.,  3,  415;  4,  234;  5,  197; 
6,  43;  Z.  f.  ph.  Ch.,  12,  117),  so  that  all  that  remains  to 
chemists  is  the  suggestion  made  in  the  Am.  Ch.  Jr.,  12, 
428 — viz.,  add  from  3  to  4  grams  of  citric  acid  to  the 
bismuth  solution,  supersaturate  the  latter  with  sodium 
hydroxide,  and  into  this  mixture  pour  the  copper  salt 
solution,  containing  a  slight  excess  of  potassium  cyan- 
ide, and  electrolyze  at  the  ordinary  temperature  with  a 
current  of  N.D100  =  0.05  ampere  and  2.7  volts.  In  9 
hours  the  bismuth  will  be  fully  precipitated  and  will 
not  contain  any  copper. 

Hollard  and  Bertiaux,  Ch.  Z.,  28,  782,  describe  a  sepa- 
ration of  bismuth  from  copper  which  is  essentially  an 
ordinary  gravimetric  precipitation  for  they  add  an  excess 
of  phosphoric  acid  to  a  boiling  solution  of  the  two  sul- 
phates. The  solution  is  allowed  to  stand  over  night. 
The  bismuth  phosphate  is  filtered  off  and  washed  with 
dilute  phosphoric  acid  (i  volume  of  acid  of  sp.  gr.  1.711 
diluted  to  20  volumes).  The  final  washing  is  per- 
formed with  ammonium  sulphydrate  and  potassium 
cyanide.  The  bismuth  phosphate  is  dissolved  in  nitric 
acid  and  the  solution  then  evaporated  in  the  presence  of 
12  c.c.  of  sulphuric  acid  until  fumes  escape.  Now  dilute 
to  300  c.c.  and  electrolyze  with  a  current  of  N.D  = 


228  ELECTRO-ANALYSIS. 

o.i   ampere.     Twenty- four  hours  will  be  necessary  for 
the  precipitation. 

10.  From  Gold.     There  is  no  recorded  electrolytic  sepa- 
ration of  these  metals. 

11.  From  Iron.     The  acid  solutions  and  conditions,  given 
on  pp.  96,  97,  98,  will  answer  in  this  case.     It  may  be 
remarked  here  that  the  deposition  of  bismuth  from  sul- 
phuric acid   solutions   containing  iron   is  attended  with 
considerable  difficulty.     The  iron  present  seems  to  exert 
an  influence  on  the  bismuth,  tending  to  hold  it  in  solution 
and  prevent  its  deposition  by  the  current.     Especially  is 
this  true  when  the  salt  used  is  a  ferric  salt.     This  ten- 
dency of  bismuth  to  be  held  in  solution  is  shown  even  in  a 
more  marked  degree  when  the  liquid   contains   besides 
ferric  alum  an  equal  quantity  of  chrome  alum.     A  cur- 
rent of  o.io  ampere  will  often  not  cause  the  slightest  pre- 
cipitation of  bismuth.     It  was  thought  that  this  behavior 
of  bismuth  could  be  used  to  separate  other  metals  from  it. 
It  was  hoped  that  the  bismuth  would  be  held  back  by  the 
iron  and  chrome  alums  and  such  metals  as  mercury,  cop- 
per, and  silver  be  deposited  from  the  solution.     These 
hopes  were  not  realized.     As  soon  as  another  metal  is 
introduced  the  condition  of  affairs  is  changed,  and  both 
the  metal  and  the  bismuth  are  precipitated.     Deposits  of 
silver,  however,  were  obtained  containing  but  very  little 
co-precipitated   bismuth.     Further    investigation    in   this 
direction  might  lead  to  some  very  interesting  and  valuable 
results. 

The  best  conditions  for  the  separation  of  bismuth  from 
iron  were  found  to  be  as  follows :  To  the  bismuth  solution 
containing  0.15  gram  of  bismuth  and  i  c.c.  of  concentrated 
nitric  acid,  add  2  c.c.  of  sulphuric  acid  (sp.  gr.  1.84),  0.5 


SEPARATION    OF    METALS BISMUTH. 


229 


gram  of  potassium  sulphate,  and  a  quantity  of  ferrous 
sulphate  or  ammonium  ferric  alum  equivalent  to  0.15 
gram  of  iron.  This  solution  should  be  diluted  to  150  c.c. 
and  electrolyzed  at  a  temperature  of  45°  C.  If  a  ferrous 
salt  is  used,  the  current  strength  should  be  0.03  ampere, 
but  if  a  ferric  salt  is  in  solution,  a  higher  current  strength 
should  be  employed, — 0.05  ampere, — the  voltage  in  both 
cases  being  2.0.  In  eight  hours  the  deposition  will  be 
complete.  The  precipitated  bismuth  is  free  from  iron 
( Kammerer) . 

In  several  cases  the  separation  was  made  in  the  presence 
of  urea  nitrate,  but  its  addition  was  no  advantage. 

RESULTS. 


TAKEN. 

FOUND 

1 

i 

TRATE. 

|| 

o 

Q 
U 

U 

H 

w 

PC 

0 

G 

u 

Q 

X 

5 

s 

| 

H 

z 

0 

M 

\\ 

o  u 

Q 

s 

D 
X 
Bi 

s 

H 

MPEK^ 

ft 

S 

o 

0 
U 

ts> 

M 

J 

H 

m 

P 

CO 

H 

& 

Grm 

Grm. 

Grm 

Grm. 

Grm 

C.c. 

C.c. 

Hours. 

°c. 

Amp. 

O.H34 

0.1429 

0.  15001 

— 

0.5 

150 

2 

8^ 

50 

0.025 

1.5 

Spiral. 

0.1431 

0.  15001 

— 

0.6 

150 

2 

71A 

45 

0.03 

2 

11 

0.1435 

o.  I5001 

•  — 

°-5 

I5° 

2 

24 

45 

0.03 

2 

" 

0.1430 

o.  I5001 

— 

0-5 

'5° 

2 

24 

45 

0.03 

1.7 

Basket. 

0.1395 

0.1394 

o.  I5001 

0-5 

0.2 

150 

2 

8 

45 

0.035 

2 

" 

0.1400 

o.  I5001 

0.5 

0.2 

150 

2 

8 

0.035 

2 

Spiral. 

0.1393 

o.  I5001 

05 

O.2 

200 

2 

8 

45 

°.°5 

2 

Gauze. 

0.1397 

O.I5002 

0.5 

150 

2 

9 

45 

O.O7 

2 

Spiral. 

0.1395 

o.i  5  oo2 

— 

I 

150 

2 

9 

45 

O.O6 

2 

" 

0.1394 

O.I5002 

— 

I 

200 

2 

8 

45 

O.O6 

2 

Gauze. 

0.1395 

o.i5oo2 

3-0 

0-5 

150 

2 

9 

45 

0.035 

2 

Spiral. 

12.  From  Lead.  Experiments  made  in  this  laboratory 
(Jr.  An.  Ch.,  7,  252)  have  demonstrated  that  the  gener- 
ally accepted  statement  that  the  metals  could  be  separated 


1  Ferrous  sulphate. 

2  Ferric  ammonium  sulphate. 


230  ELECTRO-ANALYSIS. 

in  the  presence  of  free  nitric  acid  is  not  correct.  The 
lead  dioxide  invariably  contained  bismuth.  We  are, 
therefore,  for  the  present  at  least,  without  an  electrolytic 
method  for  their  separation. 

Hollard  and  Bertiaux — B.  Soc.  Ch.,  31,  1133  (1904) 
— recommend  adding  to  the  two  nitrates  12  c.c.  of  sul- 
phuric acid  plus  the  requisite  amount  of  this  acid  to  com- 
bine with  the  two  metals,  viz.,  for  lead  0.3  c.c.  and  for 
bismuth  0.5  c.c.,  then  evaporate  until  white  fumes  arise. 
Cool.  Add  water  to  300  c.c.  and  35  c.c.  of  absolute 
alcohol.  Electrolyze  with  a  current  of  o.i  ampere  for  a 
period  of  48  hours. 

13.  From  Magnesium.     The  acid  solutions  and  conditions 
given    for   the   separation    of   bismuth    from    aluminium 
(p.  225)  will  serve  to  effect  this  particular  separation. 

14.  From  Manganese.     To  the  bismuth  solution  contain- 
ing 0.1500  gram  of  metal  and  I  c.c.  of  nitric  acid  (sp.  gr. 
1.42)  add  3  c.c.  of  sulphuric  acid  (sp.  gr.  1.84),  0.5  gram 
of  potassium  sulphate,  and  a  quantity  of  manganous  sul- 
phate equivalent  to  0.1500  gram  of  manganese.     Dilute 
this  solution  to  150  c.c.  with  water  and  electrolyze  with  a 
current  of  N.D100  =  0.025  ampere  and  2  volts,  keeping 
the  temperature  at  45°  C.     The  bismuth  will  be  deposited 
in  9  hours  in  a  beautiful  form,  free  from  manganese. 

At  first  the  solution  assumes  a  dark  red  color  due  to  the 
oxidation  of  some  of  the  manganese  into  permanganic 
acid.  After  an  hour  or  two  the  color  begins  gradually  to 
fade  away  and  the  solution  again  becomes  colorless.  A 
considerable  quantity  of  hydrated  oxide  of  manganese 
deposits  on  the  anode  during  the  electrolysis.  This  de- 
posit was  always  examined  for  bismuth,  but  in  no  case 
was  it  found  to  contain  any  of  this  metal  (Kammerer  and 
Am.  Ch.  Jr.,  8,  206). 


SEPARATION    OF    METALS BISMUTH.  2JI. 

15.  From  Mercury.     See  the  separation  of  mercury  from 
bismuth,  p.  216.  , 

1 6.  From     Molybdenum.      At     present     no     electrolytic 
method  is  know  for  this  purpose. 

17.  From  Nickel.     The  directions  recorded  on  pp.  96,  97 
for  the  determination  of  bismuth  in  acid  solutions  may  be 
followed  with  confidence  in  making  this  separation  (Am. 
Ch.  Jr.,  8,  206;  Jr.  An.  Ch.,  7,  252;  Z.  f.  anorg.  Ch.,  4, 
270). 

1 8.  From  Palladium  and  Platinum.     Separations  are  not 
known. 

19.  From  Potassium.     Follow  the  methods  given  for  the 
determination  of  bismuth  itself,  pp.  96,  97,  98. 

20.  From    Selenium.     There    is    no    existing   electrolytic 
method. 

21.  From  Silver.     Freudehberg  (Z.  f.  ph.  Ch.,  12,   108) 
uses  the  nitrates  of  the  two  metals,  adds  to  their  solution 
several  cubic  centimeters  of  nitric  acid  of  sp.  gr.  1.2  and 
from  2  to  4  grams  of  ammonium  nitrate,  then  electrolyzes 
with  a  current  having  a  potential  of  1.3  volts.     The  silver 
is  precipitated  through  the  night.     The  liquid  containing 
the  residual  bismuth  may  be  worked  for  the  determination 
of  the  bismuth  by  the  amalgam  method,  p.  96,  although  it 
would  appear  that  Freudenberg  always  determined  it  by 
evaporation  of  the  nitric  acid  solution  and  ignition  of  the 
residue,    weighing   finally   bismuth   oxide.     The    results 
obtained  by  him  are: — 

Silver  used,  0.3790  gram  ;  Bi  =  0.3080  gram 

Silver  found,  0.3793  gram  ;  Bi  =  0.3073  gram 

Silver  used,  0.2916  gram;  Bi  =  0.3080  gram 

Silver  found,  0.2914  gram;  61  =  0.3072  gram 


232  ELECTRO-ANALYSIS. 

22.  From  Sodium.     Any  one  of  the  methods  pursued  in 
the  determination  of  bismuth  when  alone  will  do  for  this 
purpose  (pp.  96,  97,  98). 

23.  From  Strontium.     See  the  separation  of  barium  from 
bismuth,  p.  225. 

24.  From  Tellurium.     There  is  no  recorded  electrolytic 
separation. 

25.  From  Tin.     The  solution  contained  0.0518  gram  of 
bismuth  and  0.1031  gram  of  tin.     To  it  were  added  5 
grams  of  tartaric  acid  and  15  c.c.  of  ammonium  hydrox- 
ide, and  the  liquid  then  diluted  to  175  c.c.  with  water 
and  electrolyzed  at  the  ordinary  temperature  with  N.D100 
=  0.02  ampere  and  1.8  volts,  during  the  night  (J.  Am. 
Ch.  S.,  15,  204). 

The  chemist  who  proposed  the  preceding  method  also 
separated  bismuth  from  a  mixture  of  arsenic,  antimony, 
and  tin.  The  solution  with  which  he  operated  contained 
0.0518  gram  of  bismuth,  0.1009  °f  arsenic,  0.1024  gram 
of  antimony,  "and  0.1031  gram  of  tin.  To  it  were  added 
8  grams  of  tartaric  acid  and  3  c.c.  of  ammonium  hydrox- 
ide, then  diluted  to  175  c.c.  with  water  and  electrolyzed 
with  a  current  of  N.D100  =  0.02  ampere  and  1.9  volts,  at 
the  ordinary  temperature.  The  precipitation  was  made 
during  the  night.  The  time  factor  can  probably  be  re- 
duced by  the  application  of  a  gentle  heat.  The  bismuth 
precipitates  rapidly  and  in  an  adherent  form. 

26.  From  Tungsten.     There  is  no  recorded  separation. 

27.  From  Uranium.     The  conditions  presented  on  p.  97 
for  the  determination  of  bismuth  in  sulphuric  acid  solu- 
tion will  serve  excellently  in  making  this  separation  (Am. 
Ch.  Jr.,  8,  206).     See  also  bismuth  from  chromium. 

28.  From  Vanadium.     There  is  no  recorded  separation. 


SEPARATION    OF    METALS LEAD.  233 

29.  From  Zinc.  The  conditions  given  in  the  determination 
of  bismuth  in  nitric  acid  (p.  96),  sulphuric  acid  (p.  97), 
and  as  amalgam  (p.  96)  will  be  found  satisfactory  in  this 
separation  (Am.  Ch.  Jr.,  8,  206;  Jr.  An.  Ch.,  7,  255). 
See  also  bismuth  from  cobalt. 


LEAD. 

The  importance  of  lead  industrially  makes  not  only  its 
accurate  determination  of  interest  and  value,  but  its  separa- 
tion from  the  metals  frequently  associated  with  it  becomes 
a  matter  of  deep  concern.  It  will  be  generally  conceded  that 
lead  is  a  metal  that  is  best  determined  by  the  electrolytic  pro- 
cedure; this  is  vastly  better  than  the  ordinary  gravimetric 
processes,  and  this,  too,  increases  the  value  of  its  separations. 

1.  From  Aluminium.     As  aluminium  is  not  precipitated 
electrolytically  from  a  nitric  acid  solution  and  the  latter  is 
especially  well  adapted  for  the  deposition  of  lead  in  the 
form  of  its  dioxide  upon  the  anode,  the  conditions  laid 
clown  upon  p.  103  will  be  found  to  answer  admirably  in 
effecting  the  present  separation. 

2.  From  Antimony.     A  purely  electrolytic  procedure  is  at 
the  present  not  known  for  the  separation  of  these  metals. 
In  the  Ch.   Z.,   19,   1142    (1895),   Nissenson  and  Neu- 
mann described  a  method  for  the  analysis  of  an  alloy  of 
antimony  and  lead,  which  deserves  attention  here.     It  is 
not  an  electrolytic  separation  in  any  sense  of  that  term, 
but  a  helpful  suggestion. 

The  finely  divided  alloy  is  brought  into  solution  with 
4  c.c.  of  nitric  acid  (sp.  gr.  1.4),  15  c.c.  of  water,  and  10 
grams  of  tartaric  acid.  Four  cubic  centimeters  of  con- 
centrated sulphuric  acid  are  added  to  the  clear  solution, 

21 


234  ELECTRO-ANALYSIS. 

which  is  then  diluted  with  water,  allowed  to  cool,  and 
filled  up  to  the  mark  of  the  ^-liter  flask.  On  filtering 
from  the  lead  sulphate,  which  has  separated,  the  filtrate 
will  contain  all  of  the  antimony.  None  will  remain  in 
the  lead  sulphate.  Remove  50  c.c.  of  the  filtrate  with  a 
pipette,  render  it  strongly  alkaline  with  caustic  soda,  add 
50  c.c.  of  a  cold  saturated  sodium  sulphide  solution,  boil, 
filter  at  once,  wash  and  electrolyze  the  hot  solution  with 
a  current  of  N.D100=  1.5-2.0  amperes.  An  hour  at  the 
most  will  be  required  for  the  deposition  of  the  antimony. 
The  lead  sulphate  should  be  digested  for  a  few  minutes 
with  ammonia  water.  This  changes  it  to  hydroxide, 
which  can  be  gradually  introduced  into  a  platinum  dish 
containing  20  c.c.  of  nitric  acid,  in  which  it  slowly  dis- 
solves. The  liquid  is  then  electrolyzed  with  the  conditions 
indicated  on  p.  103. 

3.  From  Arsenic.     Neumann   (Ch.  Z.,  20,  382)   records 
his  experience  in  attempting  to  separate  these  metals  elec- 
trolytically,  from  which  the  conclusion  may  be  deduced 
that  in  the  presence  of  arsenic  the  lead  determinations  are 
not  reliable.     They  are  too  low.     When  there  is  only  a 
fraction  of  a  per  cent,  of  arsenic  present,  the  results  can 
be  used,  although  the  time  then  necessary  for  the  complete 
precipitation  of  the  lead  as  dioxide  is  prolonged  to  an  un- 
warrantable degree.     The  experiments  of  Neumann  were 
all  conducted  in  nitric  acid  solution. 

4.  From  Barium,  Strontium,  Calcium,  Magnesium,  the 
Alkali  Metals,  Beryllium,  Cadmium,  Chromium,  Iron, 
Uranium,  Zirconium,  Zinc,  Nickel,  and  Cobalt  the  sep- 
aration of  lead  is  easily  made  by  observing  the  conditions 
given   (p.   101)   for  its  determination.     There  should  be 
from  1 5  to  20  per  cent,  of  concentrated  nitric  acid  present. 


SEPARATION    OF    METALS LEAD.  235 

The  liquid  poured  off  from  the  deposit  of  lead  peroxide 
is  changed  into  the  most  favorable  salt  for  the  precipita- 
tion of  the  particular  metal  and  the  electrolysis  proceeded 
with  in  the  usual  way. 

5.  From  Bismuth.     See  p.  229. 

6.  From  Copper.     This  separation  has  always  been  made 
in  the  presence  of  free  nitric  acid.     The  details  of  pro- 
cedure are  described  under  copper  from  lead,  p.  193. 

7.  From  Gold.     This  combination  of  metals  has  not  re- 
ceived any  attention,  apparently,  in  the  electrolytic  way 
as  the  separation  can  be  made  more  satisfactorily  in  other 
ways. 

8.  From  Manganese: — 

(a)  In  nitric  acid  solution.  It  is  well  known  thai  man- 
ganese can  be  precipitated  from  solutions  in  which  the 
quantity  of  free  nitric  acid  does  not  exceed  from  3  to  5 
per  cent.  Greater  quantities  of  the  acid  prevent  its 
appearance,  its  presence  being  made  evident  by  the  pink 
tinge  of  permanganic  acid  about  the  anode.  As  lead 
is  completely  deposited  even  in  the  presence  of  from 
15  to  20  per  cent,  of  acid,  it  would  seem  as  if  the  sepa- 
ration could  be  made  under  the  latter  conditions.  Until 
recently  it  has  not  been  undertaken.  Neumann  recom- 
mends heating  the  solution  containing  the  two  metals 
and  20  per  cent,  of  concentrated  nitric  acid  to  70°,  then 
electrolyzing  with  a  current  of  from  1.5  to  2  amperes 
and  2.5  to  2.7  volts.  It  is  absolutely  essential  to  use  hot 
solutions,  strong  currents,  and  not  too  large  quantities 
of  manganese  (0.03  gram  of  manganese  at  the  most  in 
150  c.c.  of  liquid).  When  large  amounts  are  employed 
and  the  electrolysis  prolonged  the  liquid  will  very  prob- 
ably become  turbid,  owing  to  the  separation  of  dioxide 
of  manganese  (Ch.  Z.,  20,  383). 


236 


ELECTRO-ANALYSIS. 


(b)  In  phosphoric  acid  solution.  Linn  adds  to  the  solu- 
tion of  the  two  nitrates  a  little  more  disodium  hydro- 
gen phosphate  than  necessary  for  complete  precipita- 
tion. The  phosphates  are  then  dissolved  in  an  excess 
of  pure  phosphoric  acid  (sp.  gr.  1.7)  and  the  solution 
electrolyzed  with  N.D100  —  .003  to  .006  ampere  and 
a  pressure  of  from  2  to  3  volts.  Wash  the  deposit  of 
lead  with  water,  alcohol  and  ether,  then  dry  at  100- 
110°  C.  (J.  Am.  Ch.  S.,  29,  82). 

9.  From  Mercury.     The  details  of  this  separation  are  given 
under  mercury  from  lead,  p.  220. 

10.  From  Selenium.     As  selenium  materially  affects  the 
deposition  of  lead  as  dioxide  from  a  nitric  acid  solution, 
it  may  be  of  interest  to  present  some  results  from  Neu- 
mann's experiments  (Ch.  Z.,  20,  383).    They  are  instruc- 
tive and  suggestive.     He  used  solutions  of  lead  nitrate 
containing  sodium   selenite.     The   first   experiment  was 
with  lead  alone,  the  others  contain  the  two  metals : — 


LEAD 
PRESENT. 

SELENIUM 
PRESENT. 

NITRIC 
ACID. 

LIQUID. 

TIME. 

AMPERES. 

VOLTS. 

LEAD 
FOUND. 

0.2238 

O.OOOO 

30  C.C. 

150  C.C. 

I   hr. 

0.8 

3 

0.2238 

0.2238 

o  0050 

3° 

150 

I 

0.8 

3 

0.2208 

0.2238 

0.0100 

30 

ISO 

I 

0.8 

3 

0.2156 

0.2238 

0.0200 

30 

150 

I 

0.8 

3 

0.1886 

0.2238 

0.0500 

30 

150 

1 

0.8 

3 

0.0327 

As  the  quantity  of  selenium  was  increased,  the  amount 
of  lead  dioxide  deposited  grew  less.     This  was  the  case 
with  lead  and  arsenic.     The  cathode  also  carried  a  deposit 
consisting  of  metallic  lead  and  selenium. 
ii.  From  Silver: — 

In  nitric  acid  solution.     An  example,  taken  from  a  num- 
ber made  in  this  laboratory,  will  give  the  best  condi- 


SEPARATION    OF    METALS LEAD.  237 

tions  for  carrying-  out  this  separation :  To  a  solution 
containing  0.1028  gram  of  silver  and  lead  equal  to 
0.0144  gram  of  dioxide  were  added  15  c.c.  of  nitric  acid 
of  1.3  specific  gravity.  After  dilution  to  200  c.c.  it 
was  electrolyzed  with  a  current  of  N.D100  =  o.i8  am- 
pere and  2.25  volts.  The  deposit  of  silver  weighed 
0.1023  gram  and  that  of  the  dioxide  0.0144  gram.  It 
is  probably  not  necessary  to  say  that  the  depositions 
were  simultaneous  and  that  the  precautions  described 
under  the  individual  metals  were  carefully  observed. 
It  must  be  borne  in  mind  that  silver  quite  often  separates 
in  the  presence  of  nitric  acid  both  as  peroxide  at  the 
anode  and  as  metal  at  the  cathode,  so  that  Luckow 
recommends  the  presence  of  at  least  18  per  cent,  of 
nitric  acid  and  also  introduces  several  drops  of  oxalic 
acid,  thus  hindering  the  precipitation  of  silver  dioxide 
(Jr.  An.  Ch.,  7,  252;  Z.  f.  ang.  Ch.,  1890,  345).  See 
also  Arth  and  Nicholas,  B.  S.  ch.  de  Paris  [3],  Tome 
29-30,  p.  633. 

12.  From  Tellurium.     This  separation  has  not  received 
any  attention. 

13.  From  Tin.     In  this  instance  the  usual  gravimetric  pro- 
cedure is  the  preferable  course  to  adopt  in  making  the 
separation. 

SILVER. 

The  current  has  proved  a  most  valuable  reagent  in  the 
separation   of  this   metal   from   many   others  which   occur 
associated  with  it.      The  ease  and  accuracy  of  these  various 
separations  recommend  them, 
i.  From  Aluminium.     The  conditions  given  on  p.  105  for 

the  precipitation  of  silver  from  a  nitric  acid  solution  will 

answer  for  this  separation. 


238  ELECTRO-ANALYSIS. 

In  using  the  rotating  anode  dilute  the  solution  to  125 
c.c.,  add  i  c.c.  of  nitric  acid  of  sp.  gravity  1.43  and  i  gram 
of  ammonium  nitrate,  then  electrolyze  with  N.D100  =  3 
amperes  and  3.5  volts.  The  time  will  be  fifteen  minutes 
for  a  quarter  of  a  gram  of  metal  or  more.  This  same 
procedure  will  serve  in  the  rapid  separation  of  silver  from 
cadmium,  chromium,  cobalt,  iron,  lead,  magnesium,  man- 
ganese, nickel  and  zinc  (J.  Am.  Ch.  S.,  26,  1290). 

2.  From  Antimony:— 

(ft)  In  ammoniacal  solution.  In  accordance  with  the 
suggestion  of  Freudenberg  (Z.  f.  ph.  Ch.,  12,  109),  if 
the  antimony  be  raised  to  its  highest  state  of  oxidation 
it  will  only  be  necessary  to  add  ammonium  sulphate  and 
ammonia  water  to  the  solution  of  the  combined  metals 
and  electrolyze  with  a  current  having  a  pressure  vary- 
ing from  1.2  to  1.3  volts.  The  precipitated  metal  will 
not  adhere  well  to  the  dish,  so  that  the  method  will  be 
used  only  when  special  reasons  demand  it. 

(b)  In  acid  solution.     To  the  nitric  acid  solution  add 
tartaric  acid,  after  having  converted  all  the  antimony 
into   pentoxide,   and   electrolyze   with   a   pressure  not 
exceeding  1.4  to  1.5  volts.     Freudenberg  remarks  that 
the  deposit  of  silver  is  not  well  suited  for  weighing. 

( c )  In  potassium  cyan  ide  so  hi  tion.     The  anti  mony  should 
exist  as  pentoxide.     After  adding  tartaric  acid  to  the 
cyanide  solution  ( i  gram  of  pure  potassium  cyanide  for 
every  o.i  gram  of  metal),  electrolyze  with  a  pressure 
of  from  2.3  to  2.4  volts. 

Fischer  found  procedures   (b)   and   (c)   very  satis- 
factory, Ber.,  36,  3297  and  Z.  f.  Elektrochem.,  9,  993. 

3.  From  Arsenic.     The   methods   just  described   for  the 
separation  of  silver  from  antimony  will  be  found  appli- 
cable in  this  case  (Am.  Ch.  Jr.,  12,  428). 


SEPARATION    OF    METALS SILVER.  239 

4.  From  Barium.     Follow  the  instructions  given  on  p.  105 
for  the  determination  of  silver. 

5.  From  Bismuth.     See  p.  231,  bismuth  from  silver. 

6.  From  Cadmium: — 

(a)  In  nitric  acid  solution.     To  the  solution  of  the  salts 
of  the  two  metals  add  15  to  20  c.c.  of  nitric  acid  of 
specific  gravity  1.3,  heat  to  60°,  and  electrolyze  with  a 
current  having  a  pressure  of  from  2  to  2.2  volts.     The 
silver  will  be  precipitated  and  should  be  treated  as  di- 
rected on  p.  107.     The  acid  filtrate  can,  by  the  addition 
of  an  excess  of  sodium  acetate,  be  changed  to  a  suitable 
form  for  the  deposition  of  the  cadmium.     See  p.  82. 

(b)  In  potassium  cyanide  solution.     Add   2   grams   of 
pure  potassium  cyanide  to  the  solution,  containing  o.  i— 
0.2  gram  of  each  metal,  dilute  to  125  c.c.,  heat  to  65°- 
75°,  then  conduct  a  current  of  N.D100  =  0.02-0.025 
ampere  and  2.1  volts  through  the  liquid.    The  silver  will 
be  completely  precipitated  at  the  expiration  of  from  4  to 
5  hours.    After  removing  the  liquid  from  the  precipitat- 
ing "dish  it  should  be  reduced  in  volume,  introduced  into 
a  second  weighed  platinum  dish,  and  electrolyzed  as 
directed  on  p.  81  for  the  deposition  of  the  cadmium. 

7.  From  Calcium  and  Chromium.     See  p.  237. 

8.  From  Cobalt.     An  example  will  show  the  conditions 
which  have  been  found  very  satisfactory  in  this  particular 
separation:  To  the  solution  of  the  silver  salt    (0.1024 
gram  of  silver)  were  added  o.i  gram  of  cobalt  as  nitrate 
and  2.75  grams  of  pure  potassium  cyanide.     The  liquid 
was  diluted  to  125  c.c.  with  water,  heated  to  65°  C.,  and 
electrolyzed  with   N.D100  =  0.038  ampere  and  2   volts. 
At  the  expiration  of  5  hours  the  silver  was  completely 
deposited.     It  weighed  0.1027  gram.     It  contained   no 


240  ELECTRO-ANALYSIS. 

cobalt  (J.  Am.  Ch.  S.,  21,  915).     This  procedure  is  pref- 
erable to  the  deposition  of  silver  from  a  nitric  acid  solu- 
tion. 
g.  From  Copper: — 

(a)  In  nitric  acid  solution.  Freudenberg  added  2  to  3 
c.c.  of  nitric  acid  of  1.2  specific  gravity  to  the  solution 
of  salts  of  the  two  metals,  then  electrolyzed  with  a 
pressure  of  1.3-1.4  volts,  and  a  current  of  o.i  ampere. 
The  silver  was  deposited  free  from  copper  (Z.  f.  ph. 
Ch.,  12,  107;  Berg-Hutt.  Z.  (1883),  375). 

At  the  ordinary  temperature  this  separation  will  re- 
quire 7  hours,  while  at  60°  the  precipitation  of  the 
silver  will  be  finished  in  4  hours.  The  liquid  siphoned 
off  from  the  silver,  after  the  addition  of  nitric  acid,  can 
be  electrolyzed  in  a  beaker  in  which  a  platinum  cone 
is  suspended.  The  copper  is  precipitated  on  the  cone. 
A  current  ranging  from  0.5  to  i.o  ampere  will  be  re- 
quired for  this.  The  solution  should  be  heated  to 
6o°-65°. 

The  plan  is  ideal,  but  those  who  have  attempted  to 
repeat  Freudenberg's  work  have  encountered  difficulties, 
and  naturally  modifications  of  the  procedure  have  been 
proposed.  Kuster  and  v.  Steinwehr  (Z.  f.  Elektro- 
chem.,  4,  451),  in  particular,  have  made  an  exhaustive 
investigation  of  the  precipitation  of  silver  from  nitric 
acid  and  its  separation  from  copper  in  the  presence  of 
the  latter  acid.  Their  conclusion  is  briefly  that  the 
solution  should  contain  from  i  to  2  c.c.  of  nitric  acid 
(sp.  gr.  1.4),  and  that  to  it  should  be  added  5  c.c.  of 
alcohol.  Further,  that  the  potential  of  the  electrolyte 
should  be  kept  constantly  at  1.35-1.38  volts.  An  ex- 
ample will  show  how  they  operated :  A  weighed  piece 
(0.3161  gram)  of  silver  coin  was  dissolved  in  2  c.c.  of 


SEPARATION    OF    METALS SILVER.  24! 

nitric  acid  (sp.gr.  1.4),  the  liquid  was  diluted  to  150  c.c., 
5  c.c.  of  alcohol  were  added,  and  the  solution  then  heated 
to  55°  and  electrolyzed  with  1.36  ±  o.oi  volt.  They 
obtained  0.2839  gram  of  silver  =  89.83  per  cent. 
(b)  In  potassium  cyanide  solution.  This  separation  was 
first  made  by  Smith  and  Frankel  (Am.  Ch.  Jr.,  12, 
104)  and  has  been  carried  out  over  a  hundred  times  in 
this  laboratory  by  experienced  persons  and  by  those 
who  lacked  experience,  but  in  all  cases  the  results  have 
been  most  satisfactory. 

Add  2  grams  of  pure  potassium  cyanide  to  the  solu- 
tion of  mixed  salts,  heat  to  65°,  and  electrolyze  the 
liquid  (125  c.c.)  with  a  current  of  N.D100  =  o.O3^ 
0.058  ampere  and  i.i— 1.6  volts.  The  silver  will  be 
precipitated  in  from  4  to  5  hours.  It  will,  of  course,  be 
understood  that  if  there  be  a  great  preponderance  of 
copper  over  the  silver  the  quantity  of  potassium  cyanide 
will  have  to  be  increased.  Example:  A  solution  con- 
tained 0.1066  gram  of  silver  and  0.5265  gram  of  cop- 
%  per.  Four  grams  of  pure  potassium  cyanide  were 
added,  the  liquid  was  heated  to  60°  and  electrolyzed  for 
3!  hours  with  a  current  of  N.D100  =  0.02-0.03  ampere 
and  1.2  volts.  The  silver  deposit  weighed  0.1066 
gram.  The  total  dilution  was  125  c.c. 

The  presence  of  three  or  four  metals  besides  the 
silver  also  requires  the  addition  of  more  alkaline 
cyanide  (J.  Am.  Ch.  S.,  23,  582,  also  Brunck,  Ber.,  34, 
1604;  Revay,  Z.  f.  Elektrochem.,  4,  313). 

In  the  preceding  electrolyte  it  is  easy  to  separate  sil- 
ver from  copper  when  using  a  rotating  anode.  To  the 
solution  of  the  metals  add  2  grams  of  potassium  cyan- 
ide, heat  almost  to  boiling  and  electrolyze  with  N.D100 

22 


242 


ELECTRO-ANALYSIS. 


=  0.4  to  o.i  ampere  and  2.5  volts.  Fifteen  minutes 
will  suffice  for  the  precipitation. 

To  show  how  this  procedure  may  be  applied  in  the 
rapid  analysis  of  a  coin  an  example  from  the  notebook 
of  Miss  Langness,  working  in  this  laboratory,  may  be 
here  introduced. 

A  dime  was  cleaned  and  cut  into  four  parts.  One 
part  was  then  weighed  (0.7070  gram),  dissolved  in  the 
least  possible  amount  of  nitric  acid,  the  excess  of  acid 
evaporated,  and  the  residue  dissolved  in  water  and 
diluted  to  100  c.c..  To  25  c.c.  of  this  solution  was 
added  \  gram  of  potassium  cyanide.  The  silver  was 
first  removed  with  a  low  current,  and  the  decanted 
liquid  after  evaporation  electrolyzed  for  the  copper. 
The  conditions  used  and  results  obtained  are  tabulated 
below. 


No. 

VOLTS. 

AMPERES. 

TIME.  MIN. 

WT.  OF  METAL. 

PER  CENT.  OF  METAL. 

I 

3-2.5 

.4-.  06 

35 

o.i589g.  Ag. 

89.90  percent,  silver. 

10 

5 

IO 

0.0177  g-  Cu. 

10.01    "       "    copper. 

2 

3-2.5 

.4-.  06 

45 

0.1588  g.  Ag. 

89.84   "       "    silver. 

10 

6 

IO 

0.0180  g.  Cu. 

10.  18  "       "    copper 

The  complete  analysis,  including  the  weighing  of  the 
coin  and  the  final  weighing  of  the  deposits,  required 
about  two  and  a  half  hours. 

If  two  portions  are  taken,  depositing  the  metals  to- 
gether in  the  one,  and  the  silver  alone  in  the  other,  the 
complete  analysis  can  be  made  in  an  hour  and  a  half, 
providing  two  dishes  are  available.  One  determination 
was  made  in  that  way.  The  coin  weighing  0.5638 
gram  was  dissolved  in  a  small  amount  of  nitric  acid 
(less  than  i  c.c.).  Part  of  the  excess  of  acid  was 


SEPARATION    OF    METALS SILVER.  243 

evaporated  and  a  few  drops  of  ammonia  added  to  neu- 
tralize the  remaining  excess.  Two  grams  of  potassium 
cyanide  were  then  introduced  and  the  solution  diluted 
to  100  c.c.  Twenty-five  cubic  centimeters  of  this 
solution  diluted  to  about  125  c.c.  were  electrolyzed  for 
the  silver  and  copper  combined,  and  a  second  portion 
for  the  silver  alone. 


VOLTS 

AMPERES 

TIME   MIN 

7 
2-5 

2 
.5-.  07 

18 

25 

o.  1409  combined  weight  of  Cu  and  Ag  99.94  percent, 
o.  1268  weight  of  silver                           90  oo  per  cent. 

10.  From    Gold.     No    successful    method    has    yet    been 
found.     See  Jr.  An.  Ch.,  6,  87. 

11.  From  Iron.     When  the  iron  is  present  as  a  ferrous  salt 
in  the  mixture  of  salts,  introduce  into  the  solution  3  grams 
of  potassium  cyanide,  dilute  to  100  c.c.  with  water,  heat 
to  65°,  and  electrolyze  with  a  current  of  N.D100  :=  0.04 
ampere  and  2.7  volts.     The  silver  will  be  fully  precipi- 
tated in  3  hours,  or  in  a  few  minutes  by  use  of  the  rotating 
anode. 

The  separation  of  these  metals  can  also  be  made  in  nitric 
acid  solution  by  observing  the  conditions  laid  down  on 
pp.  104,  105. 

12.  From  Lead.     Consult  p.  236,  where  the  separation  of 
lead  from  silver  is  described.     See  also  Arth  and  Nico- 
las, Ch.  N.  88,  309. 

13.  From  Lithium.     See  silver  from  barium  and  the  alka- 
line earth  metals,  p.  239. 

14.  From  Magnesium.     See  silver  from  barium,  p.  239. 

15.  From  Manganese.     See  lead  from  manganese,  p.  235. 

1 6.  From    Mercury.     There    is    no    known    electrolytic 


244  ELECTRO-ANALYSIS. 

method  for  the  separation  of  these  metals.  It  is  true  that 
both  can  be  precipitated  from  a  nitric  acid  solution  (p. 
222),  their  joint  weight  be  determined,  after  which  the 
mercury  can  be  expelled  by  heat  and  the  silver  residue 
be  reweighed. 

17.  From  Molybdenum,  Tungsten,  and  Osmium.     Fol- 
low the  conditions  recommended  as  satisfactory  in  the 
separation  of  silver  from  cobalt,  p.  239. 

1 8.  From  Nickel.     Add  1.5  gram  of  pure  potassium  cy- 
anide to  the  solution  containing  equal  amounts  of  the 
metals    (0.1-0.2  gram),  dilute  to   125  c.c.  with  water, 
heat   to    6o°— 65°,    and    electrolyze    with    a    current   of 
N.D100  =  0.02-0.03  ampere  and  a  pressure  of  1.6-2.0 
volts.     The  period  of  precipitation  is  usually  3  hours  (J. 
Am.  Ch.  S.,  21,  915). 

To  reduce  the  time  factor  use  the  rotating  anode.  To 
the  solution  of  the  salts  of  the  metals  add  1.5  gram  of 
pure  potassium  cyanide  and  electrolyze  with  a  current 
of  N.D100  — 0.4  to  0.07  ampere  and  2.5  volts.  The 
separation  will  be  finished  in  20  minutes. 

19.  From    Palladium.     The    electrolytic    separation    of 
silver  from  palladium  has  not  yet  been  made  with  any 
satisfaction. 

20.  From  Platinum.     To  the  solution  of  the  combined 
metals  add  (for  0.2  gram  of  each  metal)   1.25  gram  of 
pure  potassium  cyanide,  dilute  to   125  c.c.  with  water, 
heat  to  70°,  and  electrolyze  with  a  current  of  N.D100  = 
0.04  ampere  and   2.5   volts.     The  precipitation  will  be 
complete  at  the  end  of  3  hours  (J.  Am.  Ch.  S.,  21,  913). 

To  hasten  this  separation  use  a  rotating  anode  with 
a  current  of  N.D100  =  0.25  to  .05  ampere  and  3  volts. 
Twenty  minutes  will  suffice  for  the  deposition  of  the 
silver. 


SEPARATION    OF    METALS SILVER.  245 

21.  From  Potassium,  the  other  Alkali  Metals,  and  Alka- 
line Earth  Metals.     See  the  separation   from  'barium. 

P-  239- 

22.  From   Selenium: — 

(a)  In  cyanide  solution.    Meyer  (Z.  f.  anorg.  Ch.,  31, 
393)  pursued  a  course  in  the  determination  of  the  atomic 
weight  of  selenium,  in  which  he  electrolyzed  silver  sele- 
nite   in   cyanide   solution.     The   silver   was   precipitated 
free  from  selenium,  so  that  this  method  may  be  regarded 
as    furnishing    a    satisfactory    separation    of    the    two 
metals.     As  working  conditions  were  not  given  by  Meyer 
those  used  with  success  in  this  laboratory  will  be  here 
introduced : 

Add  to  the  solution  of  the  two  metals  3  grams  of 
potassium  cyanide,  heat  to  60°  C,  and  electrolyze  with 
a  current  of  N.D100  =0.02  ampere  and  2.5  volts.  The 
separation  will  be  finished  in  6  hours. 

(b)  In  nitric  acid  solution.     Add  i  c.c.  of  nitric  acid 
(sp.  gr.  1.43)  to  the  solution  of  the  metals,  heat  to  60° 
C.,  and  electrolyze  with  a  current  of   N.D100  =  0.015 
ampere  and  1.25  to  2  volts.     Time,  3  hours. 

23.  From  Tellurium.     In  a  cyanide  solution  this  separa- 
tion did  not  succeed. 

Add  to  the  solution  of  the  two  metals  one  cubic  centi- 
meter of  nitric  acid  (sp.  gr.  1.43),  dilute  to  150  c.c.,  heat 
to  60°  C.,  and  electrolyze  with  a  current  of  N.D100  = 
o.oi  to  0.015  ampere  and  1.25  to  2  volts.  Time,  3! 
hours. 

24.  From  Tin.     When  tin  and  silver  are  present  together, 
digest  their  sulphides  with  ammonium  sulphide,  which 
will  bring  the  tin  into  a  proper  condition  to  effect  its 
determination    electrolytically    (p.    167).     Dissolve    the 
insoluble   silver   sulphide   in   nitric   acid,   and   after   the 


246  ELECTRO-ANALYSIS. 

excess  of  the  latter  is  expelled,  add  an  excess  of  potas- 
sium cyanide  and  proceed  as  directed  on  p.  106.  The 
silver  will  be  deposited  as  a  dense  coating,  and  may  be 
washed  with  hot  water. 

This  same  course,  which  is  not  a  strict  electrolytic  pro- 
cedure, has  also  been  recommended  for  the  separation  of 
silver  when  associated  with  arsenic,  antimony,  and  tin. 

25.  From  Uranium.     See  aluminium  from  silver,  p.  237. 

26.  From  Zinc.     Add  i  gram  of  pure  potassium  cyanide 
to  the  liquid  containing  at  least  o.i  gram  of  each  metal, 
dilute  to    125   c.c.   with  water,   and  electrolyze   at  70° 
with   a   current   of   N.D100  ==  0.032-0.038   ampere   and 
2.76  volts.     The  silver  will  be   fully  precipitated  in  3 
hours.     Treat  as  described  on  p.  106  (J.  Am.  Ch.  S.,  21, 


By  using  the  rotating  anode,  in  the  presence  of  2.5 
grams  of  potassium  cyanide,  a  current  of  N.D100  =  o.3 
ampere  and  3  volts  will  precipitate  the  silver  in  twenty 
minutes. 

GOLD. 

Separations  of  gold  from  certain  metals  have  been  car- 
ried out  in  the  electrolytic  way  with  marked  success. 
As  they  may  prove  helpful,  it  was  deemed  advisable  to 
describe  them  here  in  sufficient  detail  to  make  them  gener- 
ally applicable. 

1.  From  Antimony.     Add  0.5  to  i  gram  of  tartaric  acid 
to  their  solution,  followed  by  3  to  4  grams  of  pure  po- 
tassium  cyanide;   then   electrolyze   with   the   conditions 
given  under  the  separation  of  gold  from  copper. 

2.  From  Cadmium:  — 

In  phosphoric  add  solution.  Add  40  c.c.  of  disodium 
hydrogen  phosphate  (sp.  gr.  1.028)  and  10  c.c.  of  phos- 


SEPARATION    OF    METALS GOLD.  247 

phoric  acid  (sp.  gr.  1.35)  to  the  solution  of  the  metals, 
dilute  to  125  c.c.,  heat  to  60°  C,  and  electrolyze  with 
a  current  of  N.D100  =  0.03  ampere  and  i  to  2  volts. 
Time  4  hours. 

3.  From  Cobalt. 

(a)  In  cyanide  solution.     In  the  early  experiments  made 
in  the  separation  of  these  metals  some  difficulties  were 
encountered,  so  that  it  will  be  necessary  to  follow  the 
directions,  given  below,  with  the  utmost  care.     After 
adding  4  grams  of  pure  potassium  cyanide  to  the  solu- 
tion, dilute  to   125  c.c.,  heat  to  65°,  and  electrolyze 
with   a   current   of   N.D100  =  0.05-0.08   ampere   and 
1.7-2  volts.     Before  interrupting   the  current   intro- 
duce i  c.c.  of  a  2  per  cent,  sodium  hydroxide  solution 
and  increase  the  current  to  o.io  ampere.     The  time 
necessary  to  effect  this  separation  is  usually  6  hours 
(J.  Am.  Ch.  S.,  21,  922). 

(b)  In  phosphoric  acid  solution.     Let  the  total  dilution 
of  the  solution  be  about  200  c.c.     There  should  be 
present  30  c.c.  of  disodium  hydrogen  phosphate  (sp. 
gr.  1.028)  and  6  c.c.  of  phosphoric  acid  (sp.  gr.  1.35). 
Heat  to  60°  C.     Electrolyze  with  a  current  of  N.D100 
=  0.03  to  0.04  ampere  and  a  pressure  of  from  i  to  2 
volts. 

4.  From  Copper.     The  alkaline  cyanide  solution  is  best 
adapted    for    this    separation.     To    the    liquid    contain- 
ing 0.1665  gram  °f  gold  and  a  like  amount  of  copper 
4  grams  of  potassium  cyanide  were  added.     The  solution 
was  diluted  to  250  c.c.  with  water,  heated  to  6o°-65°, 
and  electrolyzed  with  a  current  of  N.D100  =  0.05-0.08 
ampere  and    1.7-1.9   volts.     At  the  expiration   of  two 
and   one-half   hours   0.1667   gram   of   gold,    free    from 


24-8  ELECTRO-ANALYSIS. 

copper,  was  precipitated.  The  liquid  poured  off  from 
the  gold,  after  the  addition  of  an  excess  of  ammonium 
carbonate,  can  be  acted  upon  with  a  more  powerful 
current  and  the  copper  be  thus  obtained  (p.  70).  See 
J.  Am.  Ch.  S.,  21,  921 ;  J.  Am.  Ch.  S.,  26,  1268. 

5.  From  Iron. 

(a)  In   cyanide   solution.     Dissolve   pure    ferrous    am- 
monium sulphate  (—  0.1300  gram  of  iron)  in  water 
and  run  this  solution  into  a  solution  of  three  grams 
of  pure  potassium  cyanide.     Next  add  this  potassium 
ferrocyanide   solution    to    the    gold    salt,    dilute    with 
water  to  125  c.c.,  heat  to  65°  C.,  and  electrolyze  with 
a  current  of  N.  D100  -—  0.36  ampere  and  2.3  to  3  volts. 
Two  and  one-half  hours  will  serve  for  the  complete 
precipitation  of  gold  (J.  Am.  Ch.  S.,  26,  1259). 

(b)  In  phosphoric  odd  solution.     To  the  solution  con- 
taining the  two  metals  add  40  c.c.  of  disodium  hydro- 
gen phosphate   (sp.  gr.   1.028)   and   10  c.c.  of  phos- 
phoric acid  (sp.  gr.  1.35),  then  dilute  to  150  c.c.,  heat 
to  65°  C.>  and  electrolyze  with  a  current  of  N.D100  = 
0.02  to  0.08  ampere  and  i   to  2.7  volts.     Five  hours 
will  be  required  for  the  precipitation  (J.  Am.  Ch.  S,, 
26,   1266). 

6.  From  Nickel. 

•  (a)  In  cyanide  solution.  Follow  the  conditions  ob- 
served in  the  separation  of  gold  from  cobalt  (see 
above). 

(b)  In  phosphoric  acid  solution.  Follow  the  conditions 
given  for  the  separation  of  gold  from  iron  (see  above) 
in  this  electrolyte  (J.  Am.  Ch.  S.,  26,  1268). 

7.  From  Palladium.     To  their  solution  add  2  grams  of 
pure  potassium  cyanide,  dilute  to   150  c.c.   with  water, 
heat  to  65°,  and  electrolyze  for  5  hours  with  a  current 


SEPARATION    OF    METALS GOLD.  249 

of  N.D100=o.03  to  0.06  ampere  and  2.5  volts.  The 
gold  will  be  precipitated  free  from  palladium.  In  using 
the  rotating  anode  with  a  cyanide  electrolyte,  containing 
equal  amounts  of  the  two  metals,  apply  a  current  of  two 
amperes  and  six  volts.  The  gold  will  be  precipitated  in 
ten  minutes. 

8.  From    Platinum.     Add    to    the    solution,    containing 
equal  quantities  of  the  two  metals,  about   1.5  gram  of 
pure  potassium  cyanide,  dilute  to  250  c.c.  with  water, 
heat  to  70°,  and  electrolyze  for  3  hours  with  a  current 
of  N.D100  — o.oi  ampere  and  2.7  volts  (J.  Am.  Ch.  S., 
21,  923).     A  current  of  2.5  amperes  and  6  volts  will 
effect  this  separation  in  fifteen  minutes  if  the  rotating 
anode  be  employed. 

9.  From  Zinc: — 

(a)  In  cyanide  solution.     In  this  separation  the  points 
to  be  observed  are  the  quantity  of  potassium  cyanide 
(4  grams),  the  current  density,   N.D100  =  o.o6  am- 
pere,  and   the   pressure,   which   should   be   about   2.6 
volts.     The  dilution  and  other  conditions  are  similar 
to   those    followed    in    the   separation   of  gold    from 
copper,  p.  247  (J.  Am.  Ch.  S.,  21,  923). 

(b)  In  phosphoric  acid  solution.-    To  the  solution  of  the 
metals  add  30  c.c.  of  disodium  hydrogen  phosphate 
(sp.  gr.  1.028)  and  6  c.c.  of  phosphoric  acid  (sp.  gr. 
1.35).     Dilute  to   150  c.c.,  heat  to  65°  C,  and  elec- 
trolyze with  a  current  of  N.D100  =  0.2  ampere. 

It  may  be  here  stated  that  the  conditions  given  for 
the  separation  of  gold  from  copper  will  serve  just  as 
well  for  the  separation  of  gold  from  molybdenum, 
tungsten,  and  osmium.  The  conditions  observed  in 
the  precipitation  of  gold  from  a  sulphaurate  solution 


25O  ELECTRO-ANALYSIS. 

(p.  163)  can  be  used  with  the  certainty  of  good  re- 
sults in  the  separation  of  gold  from  arsenic,  molybde- 
num, and  tungsten,  while  its  deposition  from  a  phos- 
phoric acid  solution  (p.  163)  will  prove  of  value  in 
its  separation  from  zinc  and  cobalt  (Am.  Ch.  Jr.,  13, 
206). 

THE   PLATINUM   METALS. 

In  this  group  of  metals  separations  are  not  very  numer- 
ous. Further  research  is  needed  in  this  particular  direction. 
For  instance  with  platinum  there  are  lacking  separations 
from  aluminium,  antimony,  arsenic,  the  alkaline  earth  met- 
als, bismuth,  lead,  manganese,  molybdenum,  selenium,  tellu- 
rium, thallium,  tin,  tungsten,  uranium  and  vanadium.  Con- 
sequently, those  from  which  it  has  been  separated  in  the  elec- 
trolytic way  are  few :  zinc,  cadmium,  iron,  nickel  and  cobalt, 
in  acid  solution  (with  a  current  of  N.D100  =  o.O7  to  0.08 
ampere  and  1.8  to  2.0  volts),  copper  (p.  198),  gold  (p. 
249),  mercury  (p.  221)  and  silver  (p.  244). 

Platinum  may  be  separated  from  iridium  in  a  slightly 
acidulated  solution  with  a  current  of  N.D100  =  0.05  ampere 
and  1.2  volts  (Classen). 

In  the  case  of  Palladium  the  only  separations  of  it  seem 
to  be  from  copper  (p.  198),  mercury  (p.  221),  silver  (p. 
244)  and  iridium  by  the  method  given  for  its  determination 
on  p.  153. 

The  separations  of  the  metals,  comprising  the  platinum 
group,  one  from  the  other,  have  thus  far  received  scant  at- 
tention, but  from  qualitative  trials  they  promise  interesting 
results. 

The  method  given  on  p.  156  for  the  precipitation  of 
Rhodium  has  not  been  applied  to  effect  any  separations. 


SEPARATION    OF    METALS  -  ANTIMONY. 


ANTIMONY,   ARSENIC,    AND    TIN. 

Under  the  metals  which  precede  this  group  will  be  found 
the  methods  that  experience  has  shown  are  best  adapted  for 
their  separation  from  any  one  member  of  this  group.  So 
far  as  the  latter  itself  is  concerned,  much  credit  is  due 
Classen  and  his  co-laborers  for  valuable  data  upon  the 
electrolytic  separation  of  its  members. 

1.  Antimony  from  Arsenic.     The  metals,  or  compounds 
of  the  same,  are  evaporated  to  dryness  with  aqua  regia, 
the  residue  dissolved  in  2  to  3  c.c.  of  water  ;  concentrated 
sodium  hydroxide  is  added  so  that  there  will  be  2.5  grams 
of  alkali  present  in  the  liquid  and  then  80  c.c.  of  sodium 
sulphide  (sp.  gr.  1.13-1.15)  are  introduced  and  the  whole 
solution  is  diluted  to  150  c.c.,  temperature  25°-38°,  and 
electrolyzed  with  N.D100  =  1.5-1.6  amperes  and  2.1  volts 
(beginning)  to  1.45  volts  (at  end).     The  time  required 
for  the  separation  of  the  antimony  is  usually  6  hours  (Z. 
f.  Elektrochem.,  i,  291). 

Or,  to  a  solution  containing  0.1268  gram  of  antimony 
and  0.2000  gram  of  arsenic,  add  15  c.c.  of  sodium  sul- 
phide of  specific  gravity  1.18,  three  grams  of  potassium 
cyanide  and  water  to  increase  the  total  volume  of  liquid 
to  70  c.c.,  then  apply  a  current  of  6  amperes  and  4  volts 
with  the  rotating  anode.  The  antimony  will  be  com- 
pletely precipitated  in  20  minutes. 

2.  Antimony  from  Tin.     The  sulphides  (or  residue  from 
a  solution  of  the  metals)  are  placed  in  a  weighed  plati- 
num dish  and  covered  with  80  c.c.  of  sodium  sulphide 
of  specific  gravity  1.13-1.15,  to  which  are  added  2  grams 
of  sodium  hydroxide.     Dilute  to  125  c.c.  with  water,  heat 
to  57°-67°,  and  electrolyze  with  a  current  of  N.D100  = 


252  ELECTRO-ANALYSIS. 

1.45-1.50  ampere  and  0.9-0.8  volt.  The  precipitation 
will  be  complete  at  the  expiration  of  2  hours  (Z.  f. 
Elektrochem.,  I,  291).  Pour  off  the  liquid  into  a  second 
dish.  Treat  the  deposit  of  antimony  as  previously  di- 
rected (p.  172).  To  prepare  the  tin  solution  for  elec- 
trolysis, proceed  as  described  (p.  167)  for  the  conversion 
of  the  sodium  into  ammonium  sulphide  (Ber.,  17,  2245; 
18,  mo). 

This  separation  has  not  always,  in  the  hands  of  chem- 
ists, given  the  results  that  were  confidently  expected. 
There  are  disturbing  features  connected  with  it.  It  is 
not  certain  that  these  have  been  absolutely  eliminated, 
although  strenuous  efforts  have  been  put  forth  to  arrive 
at  such  a  result.  Very  recently  Ost  and  Klapproth  (Z.  f. 
ang.  Ch.,  1900,  p.  827)  conducted  experiments  in  a  cell 
provided  with  a  diaphragm  (p.  174).  These  demon- 
strated that  by  using  a  concentrated  sodium  sulphide  solu- 
tion the  current,  as  a  rule,  mainly  decomposes  the  sodium 
sulphide,  and  the  antimony,  if  the  bath  pressure  is  low, 
does  not  participate  in  the  electrolysis.  It  is  precipitated 
as  a  secondary  product  by  the  sodium  ion.  When  the 
pressure  is  great  and  the  antimony  salt  assists  in  con- 
ducting the  current,  then  the  antimony  wanders  in  the 
form  of  a  complex  anion,  SbS4,  to  the  anode.  Disturb- 
ances also  arise  from  the  commingling  of  the  anode  and 
cathode  liquids,  so  that  these  investigators  have  worked 
out  the  following  piece  of  apparatus,  to  be  used  in  this 
separation,  which  in  their  hands  has  yielded  very  satis- 
factory results.  The  sketch  (Fig.  31)  gives  a  perfect 
idea  of  their  scheme,  a  is  a  low  beaker;  the  cylindrical 
diaphragm  (a  Pukall  porous  cell),  b,  stands  in  it.  The 
anode  is  a  rod  of  carbon,  c,  placed  within  the  diaphragm- 
cell,  while  a  bent  sheet  of  platinum  or  a  platinum  gauze,  d, 


SEPARATION    OF    METALS ANTIMONY.  253 

serves  as  cathode.  The  beaker  and  cell  are  covered  with 
suitable  cover-glasses.  The  diaphragm-cell  above  the 
liquid  is  covered  with  a  suitable  rubber  ring,  e,  so  that  the 
drops  of  liquid  falling  from  the  cover-glass  are  returned 
to  the  cathode  chamber.  The  diaphragm,  thoroughly 

FIG.  31. 


cleansed,  should  always  be  preserved  under  water.  The 
anode  liquor  should  be  introduced  into  the  diaphragm-cell 
some  time  before  the  electrolysis  begins  and  the  apparatus 
should  not  be  connected  up  until  this  liquor  has  penetrated 
through  the  walls  of  the  diaphragm.  During  the  electrol- 
ysis the  level  of  the  anode  solution  should  stand  from  0.5 


254 


ELECTRO-ANALYSIS. 


to  i  cm.  higher  than  that  of  the  cathode  solution.  The 
anode  chamber  contains  from  40  to  50  c.c.,  and  the 
cathode  chamber  150  c.c.  The  total  volume  of  the  elec- 
trolytes is  about  150  c.c.  The  available  surface  of  the 
cathodes  equals  i  sq.  dm. 

To  illustrate  the  practical  working  of  this  idea,  several 
results  taken  from  Klapproth"s  doctoral  thesis  (Die 
Fallung  cles  Zinns  und  seine  Trennung  vom  Antimon 
durch  Elektrolyse,  Hannover,  1901)  may  here  be  in- 
corporated : — 

SEPARATION     OF    ANTIMONY   AND    TIN.     DIAPHRAGM     AND 
CARBON     ANODE. 


af 

A 

E 

SOLUTION  OF  NINETY  c  c.  IN 

h 

O 

H 

Q* 

u-^ 

CATHODE  CHAMBER 

H 

,1     M 

O 

D  to 

K  § 

SOLUTION  OF  FIFTY 

1 

I 

£ 

fa 

^  0 

Z 

c.c.  IN  ANODE 
CHAMBER. 

DH 

"1 

w" 
M 

|a 

K^T 

I"H 

2  « 

Z  g 

• 

£  ^ 

g 

s  z 

2  ^ 

yfi 

*" 

ZK 

H 

H 

S" 

i 

w 

K 

H  " 
2 

55 

«  H 

£ 

y.0 

W)° 

8 

PH 

b 

Q 

40 

0.1500 

0.2500 

30  Na2S 

20° 

0.08 

0.9 

0.1505 

16 

35 

0.1500 

0.2500 

30  Na2S 

20° 

0.19 

I.IO 

0.1446 

7 

60 

0.1500 

0.5000 

(  20(NH4)2S      I 
I  3o(NH4)2S04  / 

20° 

0.2 

o.5 

O.I5OO 

16 

40 

0.3000 

0.2500 

/  20(NH4)2S      \ 
\  3o(NH4)2SOj 

20° 

0.15 

1.2 

o.  2990 

7 

5o 

o.  1  500 

0.2500 

f  20(NH4)2S       ) 
\  3o(NH4)2S04  f 

20° 

I.O 

o.  H95 

16 

The  solution,  freed  from  antimony,  can  now  be  changed 
to  one  suitable  for  the  precipitation  of  the  tin  by  digesting 
it  with  ammonium  sulphate  (p.  167).  If  this  is  to  be 
done  in  the  absence  of  the  diaphragm,  then  the  latter  must 
be  removed  from  the  solution,  placed  over  the  cathode 
beaker,  and  be  washed  for  one-half  hour,  by  allowing 
water  to  run  through  it.  The  liquid  is  later  concentrated 
and  electrolyzed  (see  p.  172). 


SEPARATION    OF    METALS TIN.  255 

But  the  tin  may  be  estimated  without  removing  the 
diaphragm.  To  this  end  the  cathode  liquor  is  reduced  to 
a  volume  of  40  c.c.  and  the  anode  solution  is  renewed. 
The  precipitation  of  the  tin  is  then  made  at  70°.  As 
much  as  0.25  gram  of  the  metal  will  be  precipitated  in 
from  2  to  3  hours.  The  pressure  should  not  exceed  2 
volts. 

When  antimony,  arsenic,  and  tin  are  present  together, 
expel  the  arsenic  from  their  solution  by  the  Fischer- 
Hufschmidt  method  (Ber.,  18,  mo),  and  separate  the 
antimony  from  the  tin  as  already  described  on  page  251. 
See  also  Fischer,  Z.  f.  anorg.  Ch.,  42,  363-417. 

In  general  analysis  phosphoric  acid  is  frequently  pre- 
cipitated as  tin  phosphate.  The  latter,  of  course,  con- 
tains tin  oxide.  Dissolve  the  precipitate  in  ammonium 
sulphide.  On  electrolyzing  the  solution  the  tin  will  be 
precipitated,  and  the  filtrate  will  contain  all  of  the  phos- 
phoric acid;  this  can  be  estimated  in  the  usual  way 
(Classen).  By  observing  this  suggestion  the  determina- 
tion of  the  phosphoric  acid  in  a  separate  portion  of  the 
material  will  not  be  required. 

Tin  from  Manganese.  Dissolve  0.5  gram  of  tin  in 
a  solution  of  bromine  in  hydrochloric  acid,  neutralize  with 
ammonium  hydroxide,  add  the  solution  of  manganese  sul- 
phate and  introduce  this  mixture  into  25  c.c.  of  a  satu- 
rated ammonium  oxalate  solution.  Next  add  100  c.c.  of 
a  saturated  oxalic  acid  solution  and  electrolyze  with  a 
current  of  one  ampere  per  i  qdm.  and  a  pressure  of  2.5 
volts.  The  tin  will  be  precipitated  in  satisfactory  form. 
Puschin,  Ch.  Z.,  30,  572;  Z.  f.  Elektrochem.,  13,  153. 


256  ELECTRO-ANALYSIS. 


IRON,    MANGANESE,    NICKEL,    ZINC,    COBALT, 
ALUMINIUM,    CHROMIUM,    AND    PHOS- 
PHORIC  ACID. 

Electrolytic  methods  for  the  separation  of  these  metals 
are  neither  so  numerous  nor  so  thoroughly  worked  out  as 
with  the  metals  already  considered.     Their  separation  from 
the  heavy  metals  has  been  outlined  under  the  same,  and  it 
only  remains  to  describe  the  courses  which  may  be  pursued 
with  this  group  of  metals  when  present  together. 
i.  Iron  from  Aluminium.     Add  sufficient  ammonium  oxa- 
late  to  the  solution  of  the  salts  of  the  metals  (preferably 
not  chlorides)  so  that  it  will  contain  from  2  to  3  grams 
of  oxalate  for  each  o.i  gram  of  metal.     Dilute  to  175  c.c., 
heat   to   40°,    and   electrolyze   with   N.D100  =  1.95-1.6 
amperes  and  4.3-4.4  volts.     The  iron  will  be  precipitated 
in  two  and  one-half  hours  (Ber.,  18,  1795;  27,  2060;  Z. 
f.  Elektrochem.,  i,  292).     It  is  not  advisable  to  allow  the 
current  to  act  longer  than  is  necessary  for  the  reduction 
of  the  iron.     Towards  the  end  of  the  electrolysis  alumin- 
ium hydroxide  is  apt  to  separate  and  will  coat  the  iron 
deposit.     When  the  latter  is  dry,  this  adhering  material 
can  be  removed  with  a  handkerchief.     The  aluminium 
must  be  determined  gravimetrically.     The  separation  of 
aluminium  hydroxide  can  be  avoided  if  ammonium  or 
potassium  tartrate  ( i  gram)  or  citrate  be  added  to  the 
solution  of  the  two  metals,  and  it  be  heated  to  60 °-,  then 
electrolyzed  with  N.D100  =  i  ampere  and  4-5  volts.     It 
is  true  that  the  iron  will  probably  contain  small  amounts 
of  carbon.     These  will  not  be  excessive  and  will  not  affect 
the  results  seriously.     See  p.  141.     Consult  Hollard  and 
Bertiaux,  C.  r.,  136,  1266. 

Drown  and  McKenna  have  endeavored  to  utilize  the 


SEPARATION    OF    METALS IRON.  257 

method  described  on  p.  142  for  the  separation  of  iron 
from  other  elements.  The  conditions  favorable  for  the 
deposition  of  the  iron  they  found  unfavorable  for  its 
separation  from  manganese.  They  experienced  no  diffi- 
culty in  separating  iron  from  aluminium  or  iron  from 
phosphoric  acid.  It  is  expected  that  the  process  will  give 
equally  good  results  in  the  separation  of  iron  and  some 
other  metals  from  titanium,  zirconium,  columbium,  and 
tantalum  (Wolcott  Gibbs,  Am.  Ch.  Jr.,  13,  571 ;  see  also 
pp.  29,  57).  To  determine  iron  in  the  presence  of  alu- 
minium in  steel  they  recommend  the  following  procedure : 
"  Dissolve  5-10  grams  of  iron  or  steel  in  sulphuric  acid, 
evaporate  until  white  fumes  of  sulphuric  anhydride  begin 
to  come  off,  add  water,  heat  until  all  the  iron  is  in  solu- 
tion, filter  off  the  silica  and  carbon,  and  wash  with  water 
acidulated  with  sulphuric  acid.  Make  the  filtrate  nearly 
neutral  with  ammonia,  and  add  to  the  beaker  in  which 
the  electrolysis  is  made  about  100  times  as  much 
mercury  as  the  weight  of  iron  or  steel  taken.  The  volume 
of  the  solution  should  be  from  300  to  500  c.c.  Connect 
with  battery  or  dynamo  in  such  a  way  that  about  2 
amperes  may  pass  through  the  solution  over  night.  .  .  . 
When  the  solution  gives  no  test  for  iron,  it  is  removed 
from  the  beaker  with  a  pipette  while  the  current  is  'still 
passing."  The  aluminium  is  determined  in  this  filtrate 
(Jr.  An.  Ch.,  5,  627).  For  the  separation  of  iron  from 
titanium  and  aluminium  consult  also  Magri  and  Ercolini, 
Atti.  R.  Accad.  dei  Lincei,  Roma  [5],  16,  I.  331. 

By  modifying  the  preceding  scheme  in  accordance  with 
the  outline  given  on  p.  57,  and  observing  the  steps  and 
precautions  detailed  under  copper,  p.  77,  iron  may  be 
easily  separated  quantitatively,  with  the  aid  of  a  mercury 
cathode. 
23 


258 


ELECTRO-ANALYSIS. 


From  Vanadium.  The  details  are  best  given  in  ex- 
amples so  that  a  tabulated  series  of  results  may  be  here 
introduced : 


H 

ii 

P 

G  —  j. 

CONDITIONS. 

a  £ 

M 

*  i 

o  < 

ll 

O) 

• 

1 

| 

t/> 

o 

fc  ^ 

-    - 

M     O     tj\  (~\ 

M 

PH 

£ 

OS 

c 

5  2 

9  * 

<i  H 

&4  -^     S 

9 

M 

5 

p 

O     M 

K      HI 

s 

Q 

Hi 

Q 

K 

«« 

5~ 

H 

s 

g 

I 

O.IO56 

o.  1054 

0.1002 

12 

7 

0.4 

7 

I 

8.5 

2 

O.IO56 

0.1051 

0.1002 

13 

14 

0.6 

7 

I 

9 

3 

O.2II2 

0.2113 

0.0200 

5 

14 

0-3 

7 

I 

7-5 

4 

O.2II2 

O.2II2 

0.0200 

5 

H 

0.4 

7 

1 

7 

The  dilution  of  solution  in  each  of  these  trials  equaled 
20  cubic  centimeters. 

From  Beryllium.  From  the  readiness  with  which 
iron  may  be  separated  from  aluminium  with  the  aid  of 
a  mercury  cathode  it  was  reasonable  to  suppose  that  its 
separation  from  beryllium  could  be  made  without  diffi- 
culty. The  series  given  in  the  appended  table  sets  forth 
the  conditions  of  successful  operation.  They  appear 
just  as  they  were  carried  out : 


H 
Z    • 

Q 

M 

a 
S  2 

w 

Q 

rS» 

Q 

M 

CONDITIONS. 

i  ^ 

D  >i 

O      ^ 

O     M 

r>«Pu    * 

5 

£2 

gfia 

S  §  ^ 

"^  2  °" 

O 

m 

d 

fc  ., 

go 

3  S3 

^  on 

X  2  wQ 

H 

M 
M 

B 

H 
K 

H 

O  « 

«  z 

i  « 

M 

M 

J 

I-J 

i* 

j^li 

H 

h 

g 

o 

BH 

o 

PQ 

M 

PQ 

Cfl 

^ 

i 

0.1056 

0.1057 

0.0818 

O.O82I 

2 

7 

0-5 

7 

0.5 

6.5 

2 

o.  1056 

0.1059 

0.0818 

0.0820 

2 

14 

°-5 

7 

0-5 

6.5 

3 

0.0105 

0.0105 

0.1636 

0.1633 

2 

4/4 

0.6 

8 

0.6 

8 

4 

O.O2OO 

0.0208 

0.1636 

0.1630 

2 

H 

0.6 

8 

0.6 

8 

5 

0.2112 

0.2113 

0.0082 

0.0082 

2 

0.4 

6.5 

1.4 

7 

6 

0.2112 

O.2II2 

0.0082 

0.0083 

2 

H 

0.4 

6.5 

1.4 

7 

See  J.  Am.  Chem.  S.,  26,  1128. 


SEPARATION    OF    METALS IRON. 


259 


After  discovering  the  rapidity  with  which  metals  were 
deposited  in  a  mercury  cathode  with  the  help  of  a  rotating 
anode  (p.  72)  it  was  proposed  to  try  out  the  separation  of 
iron  in  this  way  from  other  metals  with  which  it  is  often 
associated  and  from  some  of  which  by  ordinary  gravi- 
metric methods  it  is  separated  with  difficulty.  The  speed 
of  the  anode  was  600  revolutions  per  minute.  The  metals 
were  present  either  as  sulphates  or  nitrates.  The  work- 
ing conditions  are  sufficiently  indicated  in  the  appended 
experiments. 

a.    IRON   FROM  URANIUM. 


W 
H 

ij 

D 
H) 

Q 

gj 

X 

i 

w 

fii 

u  o  6 

gjjj 

s 

«i 

Q 
2 

ft! 

O 

4* 

K  *< 

o 

S  01  M 

5  2 

g  5 

U-  *^ 

C/3   g 

PH   ^* 

u  z 

DO    II 

«  & 

o 

ft! 

C^ 

£J  ^ 

g  * 

i2 

xz  " 

3  8 

K* 

w 

^O 

0 

g 

O 

|       3 

U*1! 

s 

ft! 

as 

OS 

< 

W 

o 

H 

O.2 

0.1777 

7 

2 

3-5 

7-5 

,s 

0.1777 



O.I 

0.1777 

6 

2 

2-5 

7-5 

15 

0.1772 

—  0.0005 

O.2 

0.1777 

7 

3 

2-5-5 

7-5 

o.  1769 

—  0.0008 

O.2 

0.1777 

7 

2 

2.5-3.5 

7-5 

J5 

0.1775 

—  O.OOO2 

&.    IRON  FROM  ALUMINIUM. 


i 

Q 

Ji 

H 

o 

U        ^ 

a 

gj 

< 

2  ^ 

S  s 

^  " 

u'^o 

a 

75 

ui 

§  j 

o 

^>  ^ 

OS  <j 

o 

HH     2 

g 

S  ^ 

O   j 

S  H 

»Q  " 

K   S 

o 

cH  z 

CH 

ctf 

M   CH 

so 

M     M 

|I 

o 

3  H 

,2o 

'vJ<J 

Jg 

o 

• 

M 

DH 

<* 

^ 

O 

D       ^ 

in 

M 

0.2 

0.1777 

7 

2 

2-5 

9-7 

15 

0.1777 

:_ 

O.2 

0.1777 

7 

0 

2-4 

9-7 

15 

0.1782 

-)-O.OOO5 

O.2 

0.1777 

7 

2 

2-5 

9-7 

15 

o.  1781 

-f  O.COO4 

0-3 

0.1777 

8 

2 

2-4.5 

7-6 

15 

0.1782 

-fo.oco5 

260 


ELECTRO-ANALYSIS. 
c.    IRON  FROM   THORIUM. 


s 

a 

4 

t 

u 

a 

3 

THORIUM  NIT 
GRAM. 

IRON  PRESE 
GRAM. 

VOLUME,  c 

SULPHURIC  A 
IN  DROPS 
(30  —  i  c.c. 

CURRENT 
AMPERES 

o 

TIME. 
MINUTES 

r 

0 

M 

0.2 

0.1777 

7 

2 

2-4 

7-6 

15 

0.1777 



0.2 

0.1777 

7 

2 

3-5 

6-5 

15 

0.1777 



0-3 

0.1777 

8 

2 

3-4 

7-5 

15 

0.1777 



0.2 

0.1777 

7 

2 

3-4 

7-5    15 

0.1777 

—  O.OOOI 

d.    IRON  FROM  LANTHANUM. 


J 

Q 

£5 

£ 

u 

a 

s 

< 

*S 

H  g 

u  o  6 

£  " 

a 

-8 

2 

0  S 

O 

z 

<     M 

£2 

u 
Z 

Ha; 

i  " 

k) 

0 

SD 

r"    Z 

^  K 

sS 

^^ 

3 

x  2 

»  ^ 

> 

^ 

ZO 

o 

"3 

O 
K 

o 

D      *-" 

M 

M 

W 

0.2 

0.1220 

10 

2 

2-4 

8-6 

15 

O.I22I 

-4  o.ooo  I 

0.15 

0.1220 

10 

2 

2-4 

8-6 

15 

0.1226 

+0.0006 

0.25 

0.1220 

10 

2 

2-4 

8-6 

15 

0.1226 

+0.0006 

?.    IRON    FROM    PRASEODYMIUM. 


a 

^ 

h 

U 

a 

gj 

SH   • 

S; 

u 

y  °  u 

ii 

H 

W    (H 

o* 

C6 

O 

o  s  <: 
O  a,  as 

M 

SQ     M 

is 

O 

§    => 

zo 

0 

x  z  II 

"  s 
u 

H  « 

!5 

0 

<W 

o 

M 

O 

j  "^ 

** 

K 

h 

C/5 

W 

0.25 

0.1235 

7 

2 

2-4 

8-5 

2O 

0.1240 

+0.0005 

0-3 

0.1235 

8 

2 

3-5 

9-6 

2O 

0.1234 

—  O.OOOI 

0.1235 

8 

2 

2-4 

8-5 

20 

0.1229 

—  0.0006 

0.25 

0.1235 

7 

2 

2-4 

8-5 

20 

0.1230 

—  0.0005 

SEPARATION    OF    METALS IRON. 


261 


/.    IRON  FROM  NEODYMIUM. 


a 

grf 

h 

3 

u 
u 

•Sa'o 

UM 

4 

Q 
2 

3 

HI 
|P 

ISs 

u  < 

PL,   DC 

H 

1 

D      • 

K8<J 

gQ  - 

X2" 

•    si 
^ 

o 

si 
PJJ 

I? 

*  BS 
2O 

0 

K 

£"> 

O 

M 

0 
> 

S~& 

3      v-' 
W 

U<1 

7$ 

O 
M 

K 
W 

o.  16 

0.1235 

7 

2 

3-4 

7-5 

20 

o.  1242 

-f-  0.0007 

0.24 

0.1235 

8 

2 

3-5 

9-5 

20 

O.I236 

-f  0  0001 

0.24 

0.1235 

8 

2 

3-5 

9-7 

20 

0.1237 

-f  0.0002 

o.  16 

0-1235 

7 

2 

3-5 

9-5 

20 

0.1237 

-^0.0002 

g.    IRON    FROM    CERIUM. 


Q 

CERIUM 
SULPHATE. 
GRAM. 

IRON  PRESENT 
GRAM. 

VOLUME,  c.c 

SULPHURIC  Aci 
IN  DROPS 
(30  =  1  c  c.). 

CURRENT. 
AMPERES. 

i 

0 

TIME. 
MINUTES. 

Q 

la 

1° 

ERROR.  GRAM 

0.12 

0.1235 

8 

2 

2-4 

9-6 

20 

0.1237 

4-O.OOO2 

0.24 

0.1235 

9 

2 

2-4 

9-6 

20 

0.1236 

-f  O.OOOI 

0.36 

0.1235 

IO 

O 

2-5 

10-7 

25 

0.1230 

—  O.OOO5 

h.    IRON   FROM   ZIRCONIUM. 


a 

H 

u 

g 

ZIRCONIUM 
SULPHATE. 
GRAM. 

RON  PRESEN 
GRAM. 

VOLUME,  c. 

JLPHURIC  A( 
IN  DROPS 

(30=1  C.C.) 

h 

H 

VOLTS. 

TIME. 

MINUTES. 

2  O 

OS 

5 
I 

w 

U 

0.2 

0.1235 

7 

o 

2-4 

7-5 

2O 

o.  1238 

-1-0.0003 

0-3 

0.1235 

8 

i 

2-4 

7-5 

2O 

o.  1230 

-(-O.OOO5 

0-5 

0.1235 

10 

2 

2-5 

6-5 

25 

0.1238 

-fO.0003 

The  conditions  under  thorium  will  answer  for  the  sepa- 
ration of  iron  from  titanium  and  from  yttrium. 
J.  Am.  Ch.  S.,  25,  888;  ibid.,  27,  1547. 


262  ELECTRO-ANALYSIS. 

2.  From  Chromium.     They  can  be  separated  in  oxalate 
solution  with  conditions  like  those  given  above  for  the 
separation  of  iron  from  aluminium,  the  only  difference 
being  that  the  temperature  should  be  about  65°    (Z.  f. 
Elektrochem.,  1/292).     The  chromium  during  the  elec- 
trolysis is  converted  into  chromate.     It  must  be  deter- 
mined gravimetrically.     The  second   course,  tartrate  or 
citrate  solution,  also  lends  itself  well  to  this  separation. 
The  requisites  are  given  above  under  iron  and  aluminium. 
It  may  be  added  here  that  just  as  iron  is  separated  in 
tartrate  or  citrate  solution  from  aluminium  and  chromium, 
so  can  it  also  be  separated  from  titanium. 

3.  From  Cobalt.     Classen  (Ber.,  27,  2060)  adds  about  8 
grams   of    ammonium    oxalate   to    the    solution   of    the 
metals,  dilutes  with  water  to  120  c.c.,  heats  to  65°— 70°, 
and    electrolyzes    with    N.D100=  1.6— 2.O    amperes    and 
electrode  pressure  of  3.0-3.6  volts.     The  time  required 
for  complete  deposition  varies  from  2  to  4  hours.     The 
metals  are  precipitated  together,  their  combined  weight 
ascertained,   then   they   are   dissolved   in   acid,   and   the 
quantity  of  iron  is  found  by  titration.     The  cobalt  is  ob- 
tained by  difference. 

Vortmann  suggests  adding  3  to  6  grams  of  ammo- 
nium sulphate  and  a  moderate  excess  of  ammonium 
hydroxide  to  the  solution  of  the  metals,  then  electro- 
lyzing  with  a  current  of  N.D100  =  0.4-0.8  ampere  and 
4-5  volts.  He  remarks  that  by  contact  with  the  ferric 
hydroxide  the  deposit  of  cobalt  will  contain  traces  of 
iron,  which  can  be  fully  eliminated  by  a  second  precipi- 
tation. (See  iron  from  nickel.) 

4.  From  Manganese.     In  considering  this  separation  it 
should  be  remembered  that  objections  have  repeatedly 


SEPARATION    OF    METALS — IRON.  263 

been  offered  to  the  suggestion  of  Classen  (Ber.,  18,  1787)  ; 
hence  to  obtain  results  at  all  satisfactory  it  is  advisable 
to  carry  out  the  separation  exactly  as  given  by  this 
chemist:  "If  a  solution  of  the  double  oxalates  of  iron 
and  manganese  is  subjected  to  electrolysis,  without  the 
previous  addition  of  a  great  excess  of  ammonium  oxa- 
late  ...  it  is  impossible  to  obtain  a  quantitative  sepa- 
ration of  the  two  metals,  because  the  manganese  dioxide 
carries  down  with  it  considerable  quantities  of  ferric 
hydroxide.  The  complete  separation  of  the  metals  is 
possible  only  when  the  separation  of  the  dioxide  is  de- 
layed till  most  of  the  iron  is  precipitated."  The  elec- 
trolysis in  the  cold  is  not  favorable;  the  large  amount 
of  ammonium  carbonate,  or  ammonia  formed  in  the 
decomposition  of  the  excessive  ammonium  oxalate,  dis- 
solves the  precipitated  dioxide.  "  The  rapid  dissociation 
of  ammonium  oxalate  when  heated,  however,  gives  a 
simple  means  of  delaying,  or  entirely  preventing,  the 
formation  of  a  manganese  precipitate  during  the  elec- 
trolysis." The  solution  containing  the  two  metals  is 
treated  with  8  to  10  grams  of  ammonium  oxalate  and 
while  hot  (70°)  is  acted  upon  with  a  current  of  N.D100 
=  0.5  ampere  and  3.1-3.8  volts.  Treat  the  iron  deposit 
as  directed  on  p.  139.  Boil  the  liquid,  poured  off  from 
the  iron,  with  sodium  hydroxide,  to  decompose  the  am- 
monium carbonate  present,  after  which  add  sodium  car- 
bonate and  a  little  sodium  hypochlorite.  The  manga- 
nese is  precipitated  as  dioxide,  and  after  solution  in 
hydrochloric  acid  is  finally  weighed  as  pyrophosphate. 

Classen  mentions  that  the  method  affords  good  re- 
sults if  the  manganese  content  is  not  too  high.  In  the 
analysis  of  ferromanganese,  for  example,  it  possesses 
no  practical  value  (Ber.,  18,  1787).  Engels  has  tried 


264  ELECTRO-ANALYSIS. 

to  use  the  plan  he  describes  for  the  deposition  of  man- 
ganese (p.  135)  in  effecting  the  separation  of  the  latter 
from  iron  (Z.  f.  Elektrochem.,  2,  414),  but  it  has  been 
observed  that  while  the  manganese  was  completely  de- 
posited as  dioxide,  it  invariably  contained  as  much  as 
0.02  gram  of  iron.  See  Koster,  Ber.,  26,  2746;  Hpllarcl 
and  Bertiaux,  C.  r.,  136,  1266. 

Scholl,  working  in  this  laboratory,  separated  iron  and 
manganese  and  determined  them  simultaneously  by  the 
following  procedure :  Ten  cubic  centimeters  of  a  manga- 
nese sulphate  solution  (=0.0988  gram  of  manganese) 
were  introduced  into  a  roughened  platinum  dish.  To 
this  were  added  10  c.c.  of  a  ferric  ammonium  sulphate 
solution  (=0.0996  gram  of  iron),  5  c.c.  of  formic  acid, 
sp.  gr.  i. 06,  and  10  c.c.  of  ammonium  acetate.  A  basket 
electrode  (the  cathode)  was  then  suspended  in  the  liquid 
and  a  current  of  N.D100=i.i  amperes  and  3.9  volts 
was  allowed  to  act  for  five  hours.  The  precipitation  of 
each  metal  was  complete,  the  manganese  of  course  sepa- 
rating as  dioxide  (J.  Am.  Ch.  S.,  25,  1045). 
5.  From  Nickel.  Classen  deposits  nickel  and  iron  together 
(same  as  cobalt  and  iron)  as  an  alloy,  which  is  weighed, 
then  dissolved  in  concentrated  hydrochloric  acid,  the  iron 
oxidized  with  hydrogen  peroxide,  and  the  ferric  so- 
lution titrated  with  a  stannous  chloride  solution.  The 
current  may  vary  from  1.75  to  2.2  amperes  and  the  volt- 
age from  3.4  to  4.0.  The  temperature  of  the  liquid  is 
usually  65°-7o°.  Two  hours  will  be  sufficient  time  for 
the  precipitation  of  0.2  gram  of  the  combined  metals. 

Under  iron  from  cobalt  mention  was  made  of  a 
method  which  can  be  pursued  in  separating  the  metals 
now  under  discussion.  To  repeat,  it  consists  in  oxidiz- 


SEPARATION    OF    METALS IRON.  265 

ing  the  iron  with  bromine,  then  introducing  into  the 
solution  from  3  to  6  grams  of  ammonium  sulphate  and 
a  moderate  excess  of  ammonium  hydroxide.  From 
this  solution  the  nickel  will  be  deposited  in  from  2  to  3 
hours,  with  a  current  of  N.D100  =  0.4-0.8  ampere.  As 
in  the  case  of  the  cobalt,  traces  of  iron  will  appear  in  the 
nickel.  This  occlusion,  so  to  speak,  of  iron  has  become 
a  subject  -of  discussion  among  those  using  electro- 
lytic methods.  Neumann  (Ch.  Z.,  22,  731)  remarks 
that  it  has  tacitly  been  understood  that  the  nickel  car- 
ries down  no  iron  with  it.  Indeed,  Engels  (Thesis, 
Bern)  claims  to  have  obtained  perfectly  correct  results. 
Vortmann,  as  indicated,  and  also  Ducru  (Ch.  Z.,  21, 
780;  C.  r.,  125,  436;  B.  s.  Ch.  Paris,  17,  1881)  recom- 
mend the  solution  of  the  nickel  and  the  determination 
of  any  iron  present.  So  well  satisfied  is  Ducru  that  he 
employs  this  method  for  the  estimation  of  nickel  in 
steel,  asserting  that  the  amount  of  enclosed  iron  is  fairly 
constant  (varying  between  i  and  2  mg.),  and  that  for 
technical  or  commercial  purposes  it  may  be  ignored. 
Neumann,  on  the  other  hand,  maintains  the  absolute 
necessity  of  determining  the  amount  of  iron  co-precipi- 
tated. In  the  analysis  of  nickel  steel  and  nickel  matte 
he  proceeds  as  follows : — 

Dissolve  the  substance  in  dilute  sulphuric  acid,  and 
after  a  brief  period  introduce  hydrogen  peroxide  into 
the  solution  to  oxidize  the  carbon  and  the  iron,  thus 
obtaining  a  clear,  yellow  solution.  Now  add  ammonium 
sulphate  and  ammonium  hydroxide,  boil  and  continue 
the  addition  of  ammonium  hydroxide  to  an  excess,  then 
dilute  to  a  definite  volume.  Filter  out  100  c.c.  of  this 
solution,  mix  with  it  ammonium  sulphate  and  ammonium 
hydroxide,  dilute  to  175-200  c.c.,  and  electrolyze  the  hot 
24 


266  ELECTRO-ANALYSIS. 

liquid   with    N.D100  =  1-2   amperes    and   3.4-3.8   volts 
The  electrolysis  will  be  finished  at  the  expiration  of  from 
ij  to  2  hours. 

For  another  method  by  Vortmann  applicable  here, 
see  zinc  from  nickel  in  the  presence  of  Rochelle  salt 
(p.  268). 

6.  From  Phosphoric  Acid.     If  the  iron  has  been  precipi- 
tated from  an  oxalate  solution  (p.  139),  from  a  citrate 
solution,  or  from  an  ammoniacal  tartrate  solution,  the 
liquids  poured   off   from  the   iron  deposit  will   contain 
the  phosphoric  acid,  which  can  then  be  removed  as  am- 
monium magnesium  phosphate.     Or,  if  the  iron  phos- 
phate be  dissolved  in  sulphuric  acid  the  iron  may  be  de- 
posited in  a  mercury  cathode,  using  at  the  time  a  rotat- 
ing anode  (see  p.  143). 

7.  From  Titanium.     The  method  described  on  p.  140,  and 
also  p.  261,  with  the  conditions  given  there,  will  answer 
perfectly  in  making  this  separation. 

8.  From    Uranium.     (Ber.,    14,    2771;    18,    2483.)     In 
making  this   separation,    follow   the   directions   outlined 
on  p.  256  for  the  separation  of  iron  from  aluminium. 
The  uranium  is  precipitated  in  the  form  of  hydroxide. 
The  separation  with  the  use  of  the  mercury  cathode  and 
rotating  anode  (p.  259)  is  decidedly  preferable. 

9.  From  Zinc.     Add  to  the  solution  of  the  metals   1-3 
c.c.  of  a  solution  of  potassium  oxalate  (1:3)  and  3  to  4 
grams  of  ammonium  oxalate  and  electrolyze  the  liquid 
with  a  current  of  N.D100=  i  to  1.2  amperes.     The  zinc 
is  deposited  first,  and  no  difficulty  is  experienced,  pro- 
viding its  quantity  is  less  than  one-third  that  of  the  iron 
present.       Classen  provides  for  this  condition  by  adding 
a  weighed  amount  of  pure  ferrous  ammonium  sulphate 


SEPARATION  OF  METALS COBALT.          267 

in  excess.     Vortmann  (M.  f.  Ch.,  14,  536)  suggests  two 
methods : — 

(a)  Add   potassium   cyanide   to   the   solution   of   the 
metals  until  the  precipitate  formed  at  first  has  dissolved, 
then  introduce  sodium  hydroxide.     The  iron  is  present 
in  the  solution  as   ferrocyanide  which,   in  the  presence 
of  free  alkali,  is  not  decomposed  by  the  current.     Avoid 
too  large  an  excess  of  potassium  cyanide,  as  it  retards 
the  separation  of  the  zinc.     The  current  should  be  N.D100 
—  0.3-0.6  ampere. 

(b)  Several   grams   of   Rochelle   salt   are   introduced 
into  the  solution  of  the  metals  and  then  an  excess  of 
10-20  per  cent,  sodium  hydroxide,  after  which  the  elec- 
trolysis is  conducted  at  5o°-6o°  with  a  current  of  N.D100 
=  0.07-0.1  ampere  and  an  electrode  pressure  of  2  volts. 

1.  Cobalt  from  Manganese.     The  course  generally  recom- 
mended for  this  separation  is  precisely  like  that  given 
for  the  separation  of  iron  from  manganese.     Owing  to 
the  great  tendency  of  the  manganese,  toward  the  close 
of  the  decomposition,  to  separate  out  as  dioxide  which 
settles   on   the   cobalt   deposit,    the   method   can   hardly 
be  regarded  as  being  accurate. 

2.  From  Nickel.     To  the  acetic  acid  solution  of  the  metals 
add    10  grams   of   ammonium    sulphocyanide,    3   grams 
of  urea,  and  from  3  to  6  c.c.  of  ammonium  hydroxide  to 
neutralize  the  excess  of  acid.     Dilute  the  solution  to  300 
to  350  c.c.  and  electrolyze  with  a  pressure  of  not  more 
than  one  volt  and  0.8  ampere  at  70° -80°  C.     The  time 
of  precipitation  is  one  and  one-half  hours.     Nickel  and 
sulphur  pass  to  the  cathode,   while  the  cobalt  remains 
unprecipitated.     The  nickel  should  be  dissolved  in  acid 
and  reprecipitated  according  to  the  method  described  on 


268  ELECTRO-ANALYSIS. 

p.  126,  to  obtain  it  pure.  The  liquid  poured  off  from  the 
first  nickel  deposit  should  be  evaporated  to  dryness  several 
times  with  nitric  acid,  the  residue  taken  up  in  water, 
and  the  solution  treated  as  directed  on  p.  133  (Bala- 
chowsky,  C.  r.,  132,  1492;  also  M.  f.  Ch.,  14,  548). 
3.  From  Zinc.  Add  several  grams  of  Rochelle  salt  and 
an  excess  of  a  dilute  sodium  hydroxide  solution  to  the 
liquid  containing  the  metals.  Warm  to  65°  and  electro- 
lyze  with  N.D100  =  0.3-0.6  ampere  and  2  volts.  Usually 
there  is  a  deposit  upon  the  anode,  hence  it  is  advisable 
to  previously  weigh  the  latter  and  again  at  110°  after 
the  precipitation  is  complete  (Elektrochem.,  Z.,  i,  7). 

1.  Nickel  from  Manganese.     What  was  said  of  the  sepa- 
ration of  cobalt  from  manganese  applies  here  in  every 
particular. 

2.  From  Zinc: — 

1.  Add  4  to  6  grams  of  Rochelle  salt  to  the  solution  of 
the  two  metals,  then  a  concentrated  solution  of  sodium 
hydroxide.     Electrolyze   the  mixture   with   a   current 
of   N.D100  =  0.3-0.6   ampere.     The   precipitation   of 
the  zinc  will  be  finished  in  a  period  of  from  2  to  4 
hours.     Pour  off  the  alkaline  liquid,   wash  the  zinc 
deposit  with  water  and  alcohol;  dry  at  100°  C. 

2.  Add   10  grams  of  ammonium  sulphate,   5  grams  of 
magnesium  sulphate,  5  c.c.  of  a  saturated  solution  of 
sulphurous  acid  and  an  excess  of  25  c.c.  of  ammonia 
(sp.   gr.   0.924)    to   the   solution   containing  the  two 
metals   as   sulphates ;   dilute  to   300  c.c.   and   electro- 
lyze  at  90°   with  a  current  of  o.i   ampere.     At  the 
expiration  of  four  hours  one  to  two  cubic  centimeters 
of  the  liquid  should  not  turn  black  on  the  addition  of 
ammonium  sulphydrate.     Continue  the  electrolysis  for 


SEPARATION    OF    METALS ZINC.  269 

an  hour  longer.  Ch.  Z.,  27,  1229  (1903)  ;  Ch.  Z.,  28, 
645;  C.  r.,  137  (1903).  853;  Mid.,  138  (1904),  1605. 
Puschin  and  Trechzinsky  outline  a  method  in  the 
Z.  f.  angw.  Ch.,  17,  892,  for  the  separation  of  tin 
from  nickel,  which  may  be  regarded  as  worthy  of 
some  consideration,  although  it  in  no  wise  is  superior 
to  the  ordinary  course  of  analysis. 

i.  Zinc  from  Manganese.  A  solution  contained  0.5074 
gram  of  zinc  sulphate  and  0.1634  gram  of  manganese 
sulphate.  To  it  were  added  5  grams  of  ammonium 
lactate,  0.75  gram  of  lactic  acid,  and  2  grams  of  ammo- 
nium sulphate.  It  was  diluted  to  200  c.c.  and  electro- 
lyzed  at  2o°-25°  C.  with  a  current  of  N.D100  =  0.24-0.26 
ampere  and  3.7-3.9  volts.  In  4  hours  22.786  per  cent, 
of  zinc  was  found,  while  theory  required  22.78  per  cent. 
(Riderer,  J.  Am.  Ch.  S.,  27,789). 

Scholl  recommends  adding  to  the  solution  of  the  two 
metals  in  the  form  of  sulphates,  10  c.c.  of  formic  acid 
of  sp.  gr.  i. 06  and  5  c.c.  of  an  ammonium  formate  solu- 
tion, then  electrolyzing  with  a  current  of  i  ampere  and 
5.4  volts,  using  a  sand-blasted  dish  as  anode  and  a  basket 
shaped  cathode.  Ten  hours  are  usually  required  for  the 
separation  as  the  electrodes  are  stationary. 

The  writer  would  recommend  the  following  course  in 
separating  the  metals  of  this  group:  Separate  the  iron 
from  the  manganese,  zinc,  nickel,  and  cobalt,  by  precipi- 
tation with  barium  carbonate.  Dissolve  the  iron  precipi- 
tate in  citric  acid,  and  electrolyze  the  solution  according 
to  the  directions  given  upon  p.  140.  The  filtrate,  con- 
taining the  zinc,  manganese,  nickel,  and  cobalt,  together 
with  a  little  barium  salt,  is  carefully  treated  with  just 
sufficient  dilute  sulphuric  acid  to  remove  the  barium. 


27O  ELECTRO-ANALYSIS. 

After  filtering,  electrolyze  the  filtrate  in  a  platinum  .dish, 
connected  with  the  anode  of  a  battery,  with  a  current  of 
0.3-0.5  ampere.  A  weighed  piece  of  platinum  foil  will  an- 
swer for  the  cathode.  The  manganese  is  deposited  as 
dioxide  (p.  136)  ;  the  other  metals  remain  dissolved  and  can 
only  be  separated  by  one  of  the  usual  gravimetric  methods ; 
or  perhaps  the  suggestion  of  Vortmann  (p.  268),  for  the 
separation  of  zinc  from  nickel  and  cobalt,  would  be  appli- 
cable here,  and  these  two  might  then  be  separated  as  out- 
lined on  p.  268.  This  course  proved  quite  satisfactory  in 
the  analysis  of  the  mineral  franklinite,  where,  after  having 
obtained  the  iron  and  manganese  as  described,  the  zinc  was 
also  determined  electrolytically  in  the  liquid  poured  off 
from  the  manganese  deposit.  If  the  solution  containing 
these  two  metals  be  very  slightly  acid  with  sulphuric  acid, 
they  can  be  precipitated  simultaneously — the  zinc  at  the 
cathode,  and  manganese  dioxide  at  the  anode. 

URANIUM. 

Smith  has  called  attention  to  the  separation  of  uranium 
in  the  electrolytic  way  from  the  alkali  metals  and  from 
barium  (p.  147).  Actual  results  are  given.  It  seemed 
desirable  to  amplify  the  suggestion;  hence  the  presenta- 
tion of  the  results  given  below.  It  may  be  said  here, 
that  in  attempting  to  separate  uranium  from  nickel  and 
cobalt  no  satisfaction  could  be  obtained,  so  that  even- 
tually that  particular  line  of  experiment  was  abandoned. 
During  the  precipitation  of  the  urano-uranic  hydrate  the 
dish  should  be  \vell  covered  so  that  as  little  evapora- 
tion as  possible  occurs.  It  was  observed  that  in  case  of 
evaporation  there  was  danger  of  other  salts  separating 
upon  the  exposed  metal,  and  on  refilling  with  water  the 


SEPARATION    OF    METALS URANIUM. 


2/1 


uranium  precipitate  was  apt  to  enclose  the  same  and  thus 
carry  with  it  a  slight  impurity.  This  precaution  is  espe- 
cially necessary  in  the  separation  from  zinc  (J.  Am.  Ch. 
S.,  23,  608). 

i.  FROM     BARIUM  .(ACETATES). 


z 

z 

..  u 

:') 

". 

H" 

a  u  u 

u 

o 

t/5 

2 

i  • 

w    . 

Bl 

?fe2 

fc* 

5 
p 

j 

M 

S 

O 

W 

£j 

M 

o 

as  al 

£S 

H  g<J 

o 

2 

M 

0 

£  « 

2 

PH  ^ 

s  ^ 

MCJ   U 

^ 

u 

»O 

a! 

cc 

D 

,J 

U 

g 

0 

0 

B 

H  u  w 

PH  " 

Q 

H 

H 

M 

^ 

H 

o.  1116 

0.1  I 

o-5 

125 

70 

N.D107  =  o.02A 

2 

51^'  O.III9 

-j-0.0003 

0.1116 

O.I  I 

0-5 

!25 

70 

N.D,07=:0.04  A 

8 

51^'  0.1117 

-f  O.OOOI 

o.  1116 

O.I  I 

0.2 

125 

70 

N.D;o7  =  o.i    A 

4-54 

0.1117 

-[-O.OOOI 

2.  FROM    CALCIUM    (ACETATES). 


M 

? 

W  u 

r). 

z 

u 

o 

• 

z 

H 

tn    • 

fe 

U 

ni 

X 

o 

.- 

W   t/5 

OS  ", 

•  '  Q 

M 

g 

• 

z  '^ 

W   5 

Sjj 

'»  " 

5 

b) 

H 

W 

D  < 

£  as 

SO 

U 

3 

O 

fa  K 

<- 

^O 

5  2 

b 

W 

P 

^ 

g 

»o 

O 

a 

"s 

Q 

g 

H 

§ 

0 

§ 

& 

0 

^ 

s 

M 

o.  1116 

O.I 

0.2 

125 

70 

N.D]07=  0.025  A 

2.25 

6* 

0.1113 

-0.0003 

o.  1116 

O.  I 

0.2 

I2.S 

70 

N.D107  ^r  o  04    A 

2.2 

5^ 

0.1114 

—  0.0004 

o.  1116 

O.I 

0.2 

125 

70 

N.D107  =  o.os    A 

2.25 

4^ 

0.1113 

—0.0003 

o.  1116 

O.I 

0.2 

125 

70!  N.D]07=^  0.025  A 

2.0 

4| 

0.1115 

—  O.OOOI 

3.  FROM    MAGNESIUM  (ACETATES). 


z 

H 
Z 

w    • 

d 

| 

•gj 

U 

«  r  5 

u 

•3 

t/i 

g 

g 

H 

z 

u    . 

fa 

U 

63 

Lj 

OS 
D 

Q*   . 

M 

w  tn 

PH   S 

f-'  Q 

% 

• 

O 

Z  f> 

O 

83 

g  as 

z 

Q 

9 

&E) 
^ 

H 

ffi 

§1 

2; 

"^     V 

^  O 

r\  <J 

< 

M 

O 

fa  as 

"^ 

C^ 

t/5 

_ 

H 

K 

a 

U 

»O 

as* 

CD 

td  ^ 

os  M 

P 

U 

y 

g 

Q 

o 

o 

Z  " 

"  j- 

^J 

Ok 

as 

o 

Pu  H 

fl 

U 

H 

p 

OS 

1 

$< 

H 

0.1116 

O.I 

O.I 

125 

70 

N.D]07  =  o.o26A 

2.22 

6 

O.III5 

—  O.OOOI 

O.I  102 

O.I 

O.I 

I25 

70 

N.  D]07  =  0.05    A 

2-25   s\ 

O.IIO4 

-f  O.OQO2 

O.I  120 

O.I 

O.I 

125 

70 

N.D107==o.i5    A 

4.0     4 

O.III9 

—  O.OOOI 

272 


ELECTRO-ANALYSIS. 


4.  FROM     ZINC    (ACETATES). 


z 

8 

w  u 

So 

. 

0 

5                2 

If. 

is 

H  0 
Z  u 

U 
Z 

w 
u 

s 

h 

z 

'7J 

(H 

tf 

0 

Q" 

§  '"^ 

o 

«  x 

w  < 
tt  3; 

3< 

o 

H 
< 

x 

St 

0 

11 

z 

PH  Q 

P*O 

<*  y 

& 

K 

p 

u 

«o 

X 

°« 

Z 

"   H 

j 
Q 

i 

g 

H 

on 

o 

X 

& 

** 

N 

^ 

H 

W 

O  1  1  2O 

O  I 

O   I 

I2C 

7O 

N  D          o  021  A 

2   2C 

5 

0.  1  102 

O.2 

0.2 

1^J 
125 

70 

N.  U107  —0.017  A 

^•^3 
2.25 

6 

0.1099 

—  0.0003 

0.  1  102 

O.I 

O.I 

"5 

70 

N.D107^=o.o2    A 

2.2 

6 

O.  IIOO 

—  O.OOO2 

O.I  102 

O.I 

O.I 

125 

75 

N.  D]07  =  0.025  A 

4.4 

4* 

o.  1  103 

4  o.oooi 

O.I  102 

0.15 

O.2 

125 

75 

N.I)10T  =  o.oi    A 

2.2 

6 

0.1105 

-f  0.0003 

O.I  102 

O.2 

O.2 

125 

75 

N.D107=^o.o2    A 

2.25 

6 

0.1099 

—  0.0003 

MOLYBDENUM. 

Under  the  various  metals  conditions  have  been  given  by 
which  molybdenum  may  be  easily  separated  from  them. 
The  fact,  however,  that  the  latter  metal  can  be  readily 
deposited  in  mercury  (p.  162)  has  made  it  possible  to  sepa- 
rate it  from  vanadium,  and  yield  results  which  are  per- 
fectly satisfactory.  The  salts  employed  were  sodium  molyb- 

FROM    VANADIUM. 


MOLYBDENUM 
PRESENT  IN  GRAMS. 

MOLYBDENUM 
FOUND  IN  GRAMS 

8 

3  z 

O, 

No.  OF  CELL  USED. 

SULPHURIC  ACID 
(>PG.  1.832) 
PRESENT  IN  DROPS 

1 

H 

g 

H 

2O 
18 
18 
20 

CONDITIONS. 

t/5 

U 

a 

0, 

g 

1.6 

2 

1.6 

1-4 

d 

O 

K^OT^  Cn  AMPERES. 

s 

o 

I 

2 

3 

4 

0.0950 
0.0950 
0.1900 
0.1900 

0.0950 
0.0940 
0.1895 
0.1887 

0  1002 
0.1002 
0.0100 
0.0100 

2 

3 
2 
2 

2O 
2O 
3° 

3° 

6-5 

5 
4-5 

4-5 

5-5 

6 

5-5 

(3  hrs.) 
(3  hrs.) 
(3  hrs.) 
(3  hrs.) 

1  Neutralized  with  caustic  potash  to   15  drops  of  sulphuric  acid  and  then 
run  under  final  conditions  for  time  given. 

2  Neutralized  with  caustic  potash  to  20  drops  of  sulphuric  acid  and  then 
run  under  final  conditions  for  time  given. 


.SEPARATION    OF    METALS CHROMIUM. 


2/3 


date  and  sodium  vanaclate.  As  indicated  in  experiments 
Nos.  3  and  4  in  the  table,  it  was  found  best  to  neutralize, 
with  potassium  hydroxide,  a  portion  of  the  sulphuric  acid 
present  after  all  the  molybdenum,  but  the  last  traces,  had 
been  deposited.  Large  amounts  of  the  acid  seem  to  exert  a 
retarding  influence  on  the  final  traces  of  molybdenum.  On 
the  other  hand  the  neutralization  must  not  be  carried  too  far, 
as  an  oxide  of  vanadium  appears  at  the  anode,  when  in- 
sufficient acid  is  present.  When  the  molybdenum  is  com- 
pletely deposited  the  solution  will  be  green  in  color.  This 
may  serve  as  an  indication  for  the  interruption  of  the 
current. 

CHROMIUM. 

Since  it  is  possible  to  precipitate  this  metal  in  mercury 
(p.  144)  it  is  natural  to  pursue  this  plan  in  effecting  sepa- 
rations from  other  metals,  especially  where  these  separations 
are  an  improvement  on  earlier  procedures.  Thus,  when  in 
the  form  of  sulphates,  it  is  comparatively  easy  to  separate 
chromium  from  aluminium  by  using  the  mercury  cathode 
and  stationary  anode  as  described  on  p.  58.  The  conditions 
are  sufficiently  given  in  the  subjoined  examples. 

i.  From  Aluminium. 


< 

s 

"  "*• 

1 

< 

<!  K 

• 

J 

JJ 

oi 

CONDITIONS. 

DO 

Ho 

zO 

£  r'n 

w 

TO1 

o 

s2 

la 

s^ 

h 

s 

w 

8 

, 

«3 

m 

02    H 

K 

D  H 

5  HH 

o 

D  d  z 

H 

H 

H 

5 

X  Q 
0 

B 

D 

o 

6 
^5 

D        tt 

s 

H 

w 

s 

0 

E 

ti 

0 

5> 

0, 

h 

On 

l-4 

C/J       Q, 

I 

0.1080 

o.  1080 

0.1421 

0.1423 

I 

6 

14 

0-35 

6 

0.8 

6.5 

2 

o.io^'o 

o.  1081 

0.1421 

0.1426 

2 

4 

14 

0-3 

6 

0.8 

6.5 

3 

0.0108 

0.0107 

0.2842 

I 

6 

2 

o-3 

5-5 

0.7 

7 

4 

0.0108 

0.0107 

0.2842 

3 

5 

i/4 

°-3 

5-5 

0.85 

7-5 

5 

o.  2160 

0.2162 

0.0142 

I 

6 

24 

0.6 

6 

1.8 

7-5 

6 

0.2160 

0.2158 

0.0142 

I 

5 

14 

0.4 

8 

i 

7-5 

274 


ELECTRO-ANALYSIS. 


2.  From  Beryllium. 

A  wide  range  in  the  time  necessary  for  this  separation  is 
permissible  without  injury  to  the  deposit.  No  deleterious 
effects  are  produced  by  the  prolonged  action  of  the  current. 
The  requisite  conditions  are  sufficiently  given  in  the  follow- 
ing table: 


1            [ 

S  Z 

a  z  . 

s 

j 

a 

Q 

ur  —  «  as 

2                   CONDITIONS. 

D 

D     r.     rj5 

2  W  g 

03*  U    ,J 

O        ; 

*   H  < 

|g< 

U  0 

5   H  Id  O 

K      1      -5 

, 

as  £  * 

«  g  * 

KiD  * 

°P 

K  ^  "Q 

. 

• 
X 

H 

• 

!/3 

H 

5|£ 

CJfa 

o 

J^fe 

" 

3 

o 

E 

H) 
O 

W 

H 

3 

3 

* 

I 

o.  1080 

o.  1079 

0.0818 

I 

4 

14 

0.4 

6 

3-5 

5 

2 

o.  1080 

0.1078 

0.0818 

I 

4 

4-5 

0.3 

6 

3-5 

5 

3.  ADDITIONAL  REMARKS   ON   METAL 
SEPARATIONS. 

In  the  preceding  pages  the  greater  number  of  recorded 
separations  have  been  made  with  stationary  electrodes, 
although  it  will  be  observed  that  there  are  numerous  records 
of  such  as  have  been  conducted  with  the  help  of  the  rotating 
anode.  This  number  will  be  greatly  augmented  in  the 
course  of  time,  as  opportunity  for  further  study  in  this  direc- 
tion is  had.  That  this  field  of  investigation  is  attractive 
and  that  suggestions  of  all  kinds  are  sure  to  be  made  is 
most  certain.  While  the  writer  has  not  had  time  to  person- 
ally investigate  all  suggestions  which  have  already  been 
made  along  the  line  cited  he  feels  constrained  to  insert  at 
this  point  the  main  features  of  a  scheme  for  metal  separation 
recently  proposed  by  H.  J.  Sand.  In  doing  this  he  would 
emphasize  the  fact  that  all  separations  referred  to  by  Sand 


ADDITIONAL    REMARKS    ON    METAL    SEPARATIONS.       2/5 


FIG.  32. 


have  been  already  carried  out  after  the  plan  developed  in  this 
laboratory  for  the  rapid  precipitation  of  single  metals,  and 
are  given  full  expression  in  the  preceding  pages.  The  basal 
thought  of  Sand  is  the  "  sepa- 
ration of  metals  by  graded 
potential." 

A   description   of   the   appa- 
ratus is  as  follows : 

"  Figs,  i a,  ib,  ic  illustrate 
the  apparatus  (Fig.  32)  de- 
signed to  meet  these  require- 
ments. It  consists  of  a  pair 
of  platinum  gauze  electrodes, 
an  inner  rotating  electrode,  ic, 
and  an  outer  electrode,  ici, 
which  surrounds  it  on  all  sides 
except  the  bottom.  The  two 
are  kept  in  position  relatively 
to  each  other  by  means  of  the 
glass  tube,  ib,  which  is  slipped 
through  the  collar  A  and  the 
ring  B  of  the  outer  electrode. 
It  is  gripped  firmly  by  the  for- 
mer, but  passes  loosely  through 
the  latter.  The  hollow  platinum-indium  stem  A  of  the 
inner  electrode  is  passed  through  the  glass  tube,  in  which  it 
rotates  freely.  The  inner  electrode  is  designed  to  produce  a 
maximum  amount  of  rotation  of  the  liquid,  and  for  this 
purpose  has  a  vertical  partition,  P.  It  is  open  at  the  bottom 
and  as  open  at  the  top  as  the  requirement  of  rigidity  in  the 
construction  of  the  frame  will  allow.  The  mesh  of  the 
gauze  is  I42  per  sq.  cm.  The  gauze  of  the  outer  electrode 
almost  completely  stops  the  rotation  of  the  liquid.  While 


2/6  ELECTRO-ANALYSIS. 

the  electrolyte  is  therefore  ejected  rapidly  from  the  center 
of  the  inner  electrode  by  centrifugal  force,  it  is  continually  re- 
placed by  liquid  drawn  in  from  the  top  and  the  bottom.  So 
great  is  the  suction  thus  produced  that  when  the  electrode  is 
moving  rapidly,  chips  of  wood  or  paper  placed  on  the  surface 
are  drawn  down  to  the  top  of  the  outer  electrode.  The 
circulation  is  practically  independent  of  the  size  of  the  beaker 
employed.  As  the  outer  electrode  surrounds  the  inner  com- 
pletely, the  lines  of  flow  of  the  current  are  contained  between 
the  two,  and  even  when  strong  currents  are  employed  the 
potential  of  the  electrolyte  anywhere  outside  the  outer  elec- 
trode is  practically  the  same  as  that  of  the  layer  of  liquid  in 
immediate  contact  with  it.  This  is  a  matter  of  great  im- 
portance when  an  auxiliary  electrode  is  employed,  as  it 
enables  the  potential  difference  electrode-electrolyte  to  be 
measured  at  any  point  in  the  liquid  outside  the  outer  elec- 
trode. The  space  between  the  surfaces  of  the  two  electrodes 
is  about  3  mm.  The  weight  of  the  outer  electrode  is  about 
40  grams,  that  of  the  inner  electrode  about  28  grams.  Fig. 
33  shows  the  stand.  It  will  be  seen  that  the  beaker  con- 
taining the  electrolyte  is  always  placed  on  a  tripod  support. 
The  outer  electrode  is  gripped  by  a  V-clamp,  the  cork 
from  the  flat  side  of  which  has  been  removed  and  replaced 
by  platinum  foil  so  as  to  obtain  metallic  contact.  The  inner 
electrode  is  held  by  a  small  chuck  which  is  flexibly  attached 
to  the  pulley  from  which  the  motion  is  derived.  The  figure 
will  fully  explain  this,  as  well  as  the  mode  of  electrical  con- 
nection by  means  of  the  mercury  contained  in  the  glass  and 
rubber  tubes  C  and  F.  There  is  thus  practically  no  resist- 
ance in  the  rotating  contact,  and  no  chance  of  its  being 
affected  by  the  air  of  a  chemical  laboratory,  a  matter  espe- 
cially important  when  the  potential  difference  of  the  two  elec- 
trodes is  measured  for  the  purpose  of  separations.  All 


ADDITIONAL    REMARKS    ON    METAL    SEPARATIONS. 


movable  connections  are  made  on  the  base  of  the  stand  on 
two  sets  of  double  terminals  which  are  permanently  joined 
to  the  holders  of  the  electrodes  by  heavy  flexible  wire. 
Those  parts  of  the  stand  which  are  exposed  to  the  vapors 

FIG.  33. 


G- 


A,  Clamp  to  grip  outer  electrode;  B,  chuck  to  grip  inner  electrode; 
C,  glass  tube  rotating  in  glass  tube  D ;  E,  oil  trap  on  C ;  F,  thick 
rubber  tube;  G,  amalgamated  copper  wire  dipping  into  mercury  con- 
tained in  C  and  F ;  H,  cord  made  of  violin  string;  /,  pulley  made  of 
rubber  tube. 


278 


ELECTRO-ANALYSIS. 


from  the  electrolyte  are  painted  with  several  coatings  of 
celluloid  in  amyl  acetate.  In  order  to  reduce  the  amount  of 
platinum  required  for  the  apparatus,  attempts  were  made 


FIG.  34- 


FIG.  35. 


FIG.  34. — INNER  ELECTRODE  WITH  GLASS  FRAME.  A,  Copper  wire 
held  in  position  in  glass  stem  by  slightly  burnt  glass  tube ;  B,  C ,  mer- 
cury; D,  piece  of  gauze  fused  through  the  glass,  and,  E,  wire  forming 
connection  between  C  and  outer  gauze;  G,  partition  cut  from  micro- 
scope slide  held  in  position  by  wire  F. 

FIG.  35. — INNER  ELECTRODE,  No.  2.  Stem  and  mercury  as  in  Fig. 
34.  A,  Bulb  to  spread  out  gas  bubbles ;  B,  gauze  fused  into  glass  to 
make  connections;  C,  wire  forming  metal  surface  of  electrode;  D,  D, 
vanes  for  stirring. 

to  construct  the  frame  of  the  inner  electrode  of  glass  and  at 
the  same  time  to  retain  its  essential  features.  Fig.  34  shows 
the  result  of  these  attempts.  The  electrode  there  depicted 


ADDITIONAL    REMARKS    ON    METAL    SEPARATIONS.      279 

was  in  continual  use  for  a  month,  after  which  the  stem  broke. 
The  weight  of  platinum  was  less  than  5  grams. 

To  avoid  the  use  of  platinum,  it  might  be  possible  to 
make  the  outer  electrode  of  silver  when  it  is  used  as  the 
cathode.  It  is  probable  that  the  metals  deposited  on  it  might 
be  removed  after  electrolysis  by  the  method  of  graded  poten- 
tial, although  experiments  in  this  direction  have  not  yet  been 
made. 

The  electrodes  ic  (Fig.  34)  and  2  (Fig.  35)  are  not 
suitable  for  solutions  containing  metals  which  very  read- 
ily pass  from  one  stage  of  oxidation  to  another,  such  as 
copper  in  ammoniacal  liquids,  iron,  tin,  etc.  In  this  case, 
an  anode  with  a  smaller  oxidation  and  stirring  efficiency  is 
necessary.  The  former  is  obtained  by  making  the  surface 
of  the  electrode  much  smaller.  Fig.  35  shows  the  electrode 
which  was  designed  for  this  purpose.  It  is  made  almost 
entirely  of  glass,  the  total  weight  of  platinum  being  ij 
grams. 

The  Auxiliary  Electrode. — The  auxiliary  electrode  al- 
ways used  for  the  present  investigation  was  a  mercury- 
mercurous  sulphate-2N  sulphuric  acid  electrode.  As  an 
auxiliary  electrode  has  hitherto  not  been  employed  in  analy- 
sis, a  special  form  (Fig.  36)  suitable  for  this  purpose  was 
designed.  The  distinctive  feature  of  this  electrode  lies  in 
the  funnel  F  and  connecting  glass  tube  A  B.  It  will  be 
seen  that  the  two-way  tap  T  will  allow  the  funnel  F  to  be 
connected  with  either  half  of  the  glass  tube  A  B,  or  will  close 
all  parts  from  each  other/  The  half  A  permanently  con- 
tains the  2N-sulphuric  acid  solution  of  the  electrode.  The 
half  B,  on  the  other  hand,  is  filled  for  each  experiment  from 
the  funnel  F  with  a  suitable  connecting  liquid,  generally 
sodium  sulphate  solution.  The  end  of  B  is  made  of  thin 


28O  ELECTRO-ANALYSIS. 

tube  of  about  ii  mm.  bore,  and  is  bent  round  several  times 
to  minimize  convection,  as  will  be  seen  from  the  figure. 
While  the  electrode  is  in  use,  the  tap,  which  must  be  kept 
free  from  grease,  is  kept  closed,  the  film  of  liquid  held  round 
the  barrel  by  capillary  attraction  making  the  electrical  con- 


FIG.  36. 


nection,  but  towards  the  end  of  a  determination  a  few  drops 
are  run  out  in  order  to  expel  any  salt  which  may  have  dif- 
fused into  the  tube.  The  normal  electrode  is  held  in  a 
separate  stand  so  that  it  can  easily  be  brought  to  or  removed 
from  the  solution  undergoing  electrolysis. 

Electrical  Connections. — For  separations  by  graded  po- 
tential the  electrical  connection  must  be  made  as  shown  in 


ADDITIONAL   REMARKS    ON    METAL    SEPARATIONS.      28 1 


Fig.  37.  The  battery  is  connected  directly  to  the  two  ends 
of  a  sliding  rheostat,  the  electrolytic  cell  to  one  of  them  and 
the  slider.  It  is  manifestly  essential  that  the  sliding  con- 

FIG.  37- 

attjery 


Rheostat 


/WV\ 


^ — f  electrodes) — (Am-meterJ > 


tact  should  be  very  good.  A  rheostat  by  Ruhstrat  of 
Gottingen,  with  a  carrying  capacity  of  15  amperes  and  a 
resistance  of  2.6  ohms,  proved  very  satisfactory.  It  was 
protected  from  the  atmosphere  of  the  laboratory  by  a  coat- 
ing of  vaselin. 

The  arrangement  (Fig.  38)  adopted  for  the  measure- 
ment of  the  potential  difference  auxiliary  electrode-cathode 
is  the  one  most  usually  employed  at  the  present  time  in 
electrochemical  research.  The  electromotive  force  to  be 
measured  is  balanced  against  a  known  electromotive  force 
by  means  of  a  capillary  electrometer.  The  known  elec- 
tromotive force  is  drawn  from  a  sliding  rheostat,  the  ends 
of  which  are  connected  with  one  or  two  dry  cells.  The 
value  of  the  E.  M.  F.  is  read  directly  on  a  delicate  volt- 
meter (range,  1.5  volts).  For  potential  difference  greater 
than  1.5  volts  a  Helmholtz  T  volt  cell  was  interposed  be- 
tween the  auxiliary  electrode  and  the  rheostat.  The  ar- 
25 


282 


ELECTRO-ANALYSIS. 


rangement  allows  the  voltage  to  be  measured  almost 
instantaneously,  a  matter  of  great  importance  in  the  present 
case.  Owing  to  the  very  great  advances  made  in  recent 
years  in  the  construction  of  quadrant  electrometers  and  their 
adjuncts,  it  seems  probable  that  an  electrometer  might  be 
permanently  fitted  up  in  such  a  manner  as  to  be  used  as 
a  direct-reading  electrostatic  voltmeter  (range  required,  i 
volt;  sensitiveness,  i  centivolt).  If  this  were  the  case  it 

FIG.  38. 


athode 


Electrometer     Auxiliar 

electrode. 


would  become  as  simple  a  matter  to  read  the  potential 
difference  between  the  cathode  and  the  electrolyte  as  that 
between  the  cathode  and  the  anode. 

Method  of  Carrying  out  an  Experiment. — Where  not 
especially  stated  to  the  contrary,  the  metal  was  always  de- 
posited on  the  outer  electrode.  To  carry  out  an  experiment 
the  cathode,  anode,  and  auxiliary  electrode  are  placed  in 
position,  the  electrolyte  is  heated  to  the  required  tempera- 
ture and  covered  with  a  set  of  clock  glasses  having  suitable 
openings  for  the  electrodes.  For  the  purpose  of  a  sepa- 
ration the  current  is  usually  started  at  about  3-4  amperes 


ADDITIONAL    REMARKS    ON    METAL    SEPARATIONS.       283 

and  the  potential  of  the  auxiliary  electrode  noted.  As  a 
rule  this  is  only  slightly  above  the  equilibrium  potential. 
The  current  is  then  regulated  so  that  the  potential  of  the 
electrode  may  remain  constant.  When  no  by-reactions 
take  place  the  current  falls  to  a  small  residual  value  (gener- 
ally about  0.2  ampere),  as  the  metal  to  be  separated  dis- 
appears from  the  solution.  The  auxiliary  electrode  is  then 
allowed  to  rise  o.i  to  0.2  volt,  according  to  the  metal. 

It  is  obviously  a  matter  of  great  importance  to  know 
when  all  the  metal  has  been  deposited.  Under  the  condi- 
tions just  assumed  the  amount  deposited  per  unit  of  time 
may  be  taken  as  roughly  proportional  to  the  amount  still 
in  solution.  This  being  so,  it  follows  that  the  amount  in 
solution  will  decrease  in  geometrical  ratio  during  successive 
equal  intervals  of  time.  If  we,  therefore,  make  the  safe 
assumption  that  the  concentration  of  the  metal  has  fallen 
to  under  i  per  cent,  of  its  original  value  in  the  time  during 
which  the  potential  and  the  current  have  been  brought  to 
their  final  value,  it  is  clear  by  continuing  the  experiment 
half  as  long  again,  the  concentration  of  the  metal  will  fall 
to  under  o.i  per  cent.,  so  that  the  deposition  can  then  be 
considered  finished. 

In  cases  where  by-reactions  occur,  the  current  does  not 
fall  to  zero,  but  it  generally  attains  a  constant  value  which 
allows  one  to  see  when  all  the  metal  has  been  removed.  In 
certain  cases,  the  absence  of  the  latter  can  be  roughly  tested 
for  chemically,  and  by  continuing  the  experiment  for  about 
half  as  long  again  as  this  reaction  demands,  the  metal  may 
be  safely  assumed  to  have  been  deposited  completely.  This 
method  may  be  adopted,  for  example,  in  the  separation  of 
lead  from  cadmium,  the  former  being  roughly  tested  for 
by  sulphuric  acid.  If  none  of  these  methods  is  available, 
the  metal  must  be  deposited  to  constant  weight  or  else  the 


284  ELECTRO-ANALYSIS. 

separation  must  be  carried  out  under  very  carefully  defined 
conditions  for  a  length  of  time  proved  more  than  sufficient 
by  previous  experiment. 

Interrupting  an  Experiment. — A  short  time  before 
completing  the  analysis,  the  inside  of  the  tube  6,  the  sides 
of  the  beaker,  and  the  clock  glasses  are  washed  by  the  aid 
of  a  wash-bottle  and  a  few  drops  of  liquid  run  out  of  the 
connecting  limb  of  the  auxiliary  electrode.  To  interrupt 
the  experiment,  the  auxiliary  electrode  and  the  clock  glasses 
are  removed,  the  tripod  is  then  taken  from  under  the 
beaker  and  the  latter  lowered  until  the  surface  of  the 
liquid  is  just  below  the  outer  electrode.  During  this  time 
the  latter  is  washed.  The  stirrer  is  now  stopped  before 
lowering  the  beaker  any  further.  The  latter  is  then  re- 
placed by  a  slightly  larger  one,  the  tripod  put  back  and  the 
electrode  again  washed.  It  is  then  disconnected,  shaken, 
dipped  first  into  a  jar  containing  alcohol,  shaken,  then  into 
another  containing  ether,  and  then  dried  for  about  a  minute 
over  a  Bunsen  burner.  The  collar  A  is  carefully  dried  by 
a  silk  cloth  before  weighing.  The  remaining  liquid  is 
washed  into  the  larger  beaker  and  is  then  ready  for  the 
deposition  of  the  next  metal. 

When  only  one  metal  is  contained  in  the  solution  under- 
going analysis,  it  is  simpler  to  stop  the  stirrer,  take  away 
the  beaker,  and  replace  it  by  two  successive  ones  containing 
distilled  water.  In  both  cases  the  current  is  left  on  during 
the  process  of  interruption. 

The  beaker  in  which  the  first  deposition  of  a  separation 
is  carried  out  was  only  slightly  wider  than  the  electrode 
and  the  amount  of  the  liquid  roughly  85  c.c.  In  the  second 
separation  the  amount  was  usually  130  c.c.  and  so  on. 

The  rate  of  stirring  varied  very  considerably  from  one 


DETERMINATION    OF    THE    HALOGENS.  285 

experiment  to  another  without  greatly  affecting  the  result. 
It  may  be  taken  as  having  been  between  the  limits  of  300 
and  600  revolutions  per  minute."  Sand,  J.  Ch.  S.  (Lon- 
don), 91,  374. 

Consult  also  A.   Fischer,   Z.    f.   Elektrochem.,   13,  469; 
Z.  f.  angw.  Ch.,  20,  134  (1907). 


4.  DETERMINATION  OF  THE  HALOGENS 
IN  THE  ELECTROLYTIC  WAY. 

LITERATURE. — Whit  field,  Am.  Ch.  Jr.,  8,  421;  Vortmann,  Elek- 
troch.  Z.,  i,  137;  2,  169;  E.  Miiller,  Ber.,  35  (1902),  950;  Specketer, 
Z.  f.  Elektrochem.,  4,  539;  With  row,  J.  Am.  Ch.  S.,  28,  1356. 

Whitfield  proceeds  as  follows :  The  silver  halide  is  col- 
lected in  a  Gooch  crucible  and  dried  directly  over  a  low 
Bunsen  flame.  After  weighing  it  is  dissolved  by  intro- 
ducing the  crucible  and  asbestos  into  a  concentrated  po- 
tassium cyanide  solution.  The  silver  is  then  deposited  in 
a  platinum  dish  of  100  cm2  surface  with  a  current  of  0.07 
ampere.  It  is  not  advisable  to  work  with  more  than  2 
grams  of  silver  halide. 

Vortmann  has  developed  an  electrolytic  scheme  for  the 
direct  determination  of  the  halogens.  As  he  has  given  the 
most  attention  to 'iodine,  its  method  of  estimation  will  be 
presented  here. 

To  the  aqueous  solution  of  potassium  iodide  were  added 
several  grams  of  Seignette  salt  and  16-20  c.c.  of  a  10  per 
cent,  solution  of  sodium  hydroxide.  The  liquid  was  then 
diluted  to  150  c.c.  and  placed  in  a  crystallizing  dish  or  in 
a  platinum  dish.  If  the  first  was  used,  then  a  platinum 
disk,  5  cm.  in  diameter,  was  made  the  cathode,  whereas 
in  the  second  instance  the  dish  itself  became  the  cathode, 


'286  ELECTRO-ANALYSIS. 

the  anode  being  a  circular  plate  of  pure  silver,  5  cm.  in 
diameter,  or  a  plate  of  platinum  of  like  size,  coated  with 
silver.  The  electrolysis  was  made  with  a  current  of  0.03- 
0.07  ampere  and  2  volts.  It  was  found  expedient,  after 
several  hours,  to  replace  the  anode  coated  with  silver 
iodide  with  another,  and  the  electrolysis  was  continued 
until  the  anode  ceased  to  increase  in  weight.  This  change 
in  anodes  is  absolutely  necessary  when  the  quantity  of 
iodine  exceeds  0.2  gram.  The  iodine  may  exist  as  iodide 
or  iodate.  The  alkaline  tartrate  is  introduced  to  prevent 
the  silver  iodide  from  becoming  detached. 

#.  Determination  of  Iodine  in  the  Presence  of  Bromine 
and  Chlorine. 

The  method  is  based  on  the  fact  that  an  iodide  in  the 
presence  of  a  soluble  chromate  in  alkaline  solution  is  oxi- 
dized to  iodate  at  a  pressure  insufficient  for  the  conversion 
of  bromides  and  chlorides  into  their  corresponding  oxy- 
salts.  The  iodate  produced  is  estimated  by  titration  with 
thiosulphate,  and  the  quantity  of  thiosulphate  used  by  the 
known  amount  of  chromate  present  is  then  deducted.  Chro- 
mate, even  in  small  amounts  prevents  reduction  at  the 
cathode.  Further,  periodate  is  not  produced.  It  is  neces- 
sary always  to  platinize  anew  the  platinum  cathode.  A 
pressure  of  1.6  volts  does  not  form  bromate  in  a  o.i  to 
o.oi  normal  solution,  while  all  of  the  iodine  is  changed  to 
iodate.  The  following  solutions  were  used  in  the  analysis : 

1.  A  potassium  chromate  solution,  of  which  i  cubic  centi- 

meter =10.6  c.c.   i/ioo  N  thiosulphate  solution. 

2.  Normal  caustic  potash. 

3.  Solution  of  potassium  iodide,  of  which  i  cubic  centi- 

meter  =  9.13    cubic    centimeters    i/ioo    N    silver 
nitrate  solution. 


DETERMINATION    OF    THE    HALOGENS.  28/ 

In  determining  iodine  in  the  absence  of  the  other  halo- 
gens mix:  2  cubic  centimeters  of  solution  i;  I  cubic  centi- 
meter of  solution  2;  10  cubic  centimeters  of  solution  3  and 
90  cubic  centimeters  of  water.  Electrolyze  for  a  peroid 
of  twenty  hours  with  a  pressure  of  from  1.6  to  1.61  volts. 
Titration  with  sodium  hyposulphite  solution  gave  0.11594 
gram  and  0.11632  gram  of  iodine  instead  of  0.1158  gram. 

In  the  presence  of  chlorine,  use : 

2  cubic  centimeters  of  solution  i 
i   cubic  centimeter   of   solution   2 

1  cubic  centimeter  of  solution  3  and 

100  cubic  centimeters  of  a  saturated  sodium  chloride  solution. 
Time  20  hours,  Volts   1.59   to    1.60. 
Result:  0.01163  and  0.01167  instead  of  0.1158. 

In  the  presence  of  bromine  use: 

2  cubic  centimeters  of  solution   i 
i   cubic  centimeter  of  solution  2 

i   cubic  centimeter  of  solution  3  and 

100  cubic  centimeters  of  a  normal  potassium  bromide  solution. 
Time,   22   hours.     Pressure,    1.6   to    1.61    volts. 
Results:  0.01158  and  0.01170  instead  of  0.01158. 

Test  the  reagents  beforehand  with  potassium  iodide  and 
sulphuric  acid  to  ascertain  whether  they  liberate  iodine. 
This  often  occurs  with  the  alkali  solutions  of  trade.  The 
anode  must  be  wholly  immersed  in  the  solution,  because 
if  iodine  is  separated  directly  at  the  surface,  it  readily 
vaporizes.  The  point  of  contact  of  the  conducting  wire 
with  the  solution  should  be  covered  with  glass.  Alkaline 
earths  should  be  absent. 

b.  Separation  of  the  Halogens. 

Metals  have  been  separated  by  graded  potential  (Kiliani, 
Freudenberg,  etc.).  This  principle  has  been  applied  re- 
cently to  the  halogens.  In  the  hands  of  Specketer  good 

N 


288  ELECTRO-ANALYSIS. 

results  have  been  obtained.  The  electrolysis  is  carried  out 
in  sulphuric  acid  solution  of  normal  concentration.  The 
method  of  conducting  the  experiment  is  briefly  as  follows : 
Use  a  Giilcher  thermopile.  It  possesses  superior  advan- 
tages for  this  particular  kind  of  work,  as  constancy  of 
current  is  an  absolute  necessity.  The  pressure  of  the  form 
used  by  Specketer  was  three  volts.  The  vessel  in  which 
the  electrolysis  is  performed  should  be  narrow  and  tall, 
something  like  a  measuring  cylinder,  so  that  nothing  is 
lost  by  spattering,  occasioned  by  conducting  hydrogen 
through  the  electrolyte  during  the  analysis,  and  in  order 
that  the  washing  of  the  anode  may  be  directly  done  in  the 
cylinder,  the  latter  should  be  closed  with  a  cork,  carrying 
the  cathode  of  sheet  platinum  and  an  anode  of  silver  gauze, 
and  sufficiently  large  to  permit  of  the  passage  of  a  gas 
delivery  tube  through  it.  The  hydrogen  finds  its  exit  im- 
mediately back  of  the  cathode  plate.  A  voltmeter  should 
be  in  circuit.  The  conclusion  of  the  analysis  is  indicated 
by  a  delicate  Edelmann  galvanometer  so  arranged  that  it 
can  readily  be  thrown  in  or  out  of  the  circuit.  The  salts 
used  were  pure  potassium  chloride,  bromide  and  iodide. 

i.  Separation  of  Iodine  from  Chlorine. 

PRESSURE  =.  0.13  volt. 

a.  IODINE  USED.  b.  IODINE  FOUND. 

0.29087  gram  0.2992  gram 

0.2394     gram  0.2386  gram 

0.0481     gram  0.0480  gram 

0.1543     gram  0.1532  gram 

•  When  the  iodine  was  completely  precipitated,  the  current 
was  interrupted,  the  anode  washed  off  in  the  cylinder  and 
then  dried  at  120°.  The  chlorine  was  determined  in  the 
residual  liquid  by  the  Volhard  method. 


DETERMINATION    OF    NITRIC    ACID.  289 

2.  Separation  of  Bromine  from  Chlorine. 

PRESSURE  =  0.35  volt. 

a.  BROMINE  PRESENT.  b.  BROMINE  FOUND. 

0.19437  gram  0.1940  gram 

0.2735     gram  0.2736  gram 

0.1962     gram  0.1958  gram 

0.1899     gram  0.1906  gram 

The  chlorine  was  again  determined  volumetrically. 
3.  Separation  of  Iodine  from  Bromine. 

PRESSURE  —  0.13  volt. 

a.  IODINE  PRESENT.  b.  IODINE  FOUND. 

0.1706  gram  0.1685  gram 

0.1636  gram  0.1610  gram 

0.2029  gram  0.2036  gram 

It  should  be  constantly  borne  in  mind  that  to  make  these 
separations  successfully  air  must  be  absolutely  excluded, 
the  source  of  current  must  be  constant  and  a  definite  acid 
concentration  must  be  maintained. 


5.  DETERMINATION  OF  NITRIC  ACID  IN 
THE  ELECTROLYTIC  WAY. 

LITERATURE. — Vortmann,    Ber.,    23,    2798;    East  on,    J.    Am.    Chem. 
S.,  25,  1042  ;  I  ngham ,  J.  Am.  Ch.  S.,  26,  1251. 

To  the  solution  of  the  nitrate,  in  a  platinum  dish,  add  a 
sufficient  quantity  of  copper  sulphate.  Acidulate  the 
liquid  with  dilute  sulphuric  acid  and  electrolyze  with  a  cur- 
rent of  o.i  to  0.2  ampere.  When  the  deposition  of  the 
copper  is  completed,  pour  off  the  liquid,  reduce  it  to  a  small 
volume,  and  distil  off  the  ammonia  in  the  usual  manner. 
The  quantity  of  copper  sulphate  added  should  be  determined 
by  the  quantity  of  nitric  acid  present.  If  potassium  nitrate 
is  the  salt  undergoing  analysis,  add  half  of  its  weight  in 
copper  sulphate. 
26 


290-  ELECTRO-ANALYSIS. 

Easton  gave  the  following  as  satisfactory  conditions, 
when  using  stationary  electrodes :  an  equal  weight  of  copper 
nitrate  and  copper  sulphate,  30  c.c.  of  sulphuric  acid  of 
specific  gravity  1.062,  a  dilution  of  150  c.c.,  a  platinum 
anode,  a  cathode  of  lead  or  copper,  or  a  platinum  dish  of 
200  c.c.  capacity,  0.15  to  3  amperes,  3  to  8  volts,  and  one 
and  a  quarter  to  eight  and  one  half  hours. 

The  Rapid  Determination  of  Nitric  Acid  With  the  Use 
of  a  Rotating  Anode. 

This  method  has  been  most  carefully  elaborated  by  Leslie 
H.  Ingham  in  this  laboratory.  The  results  of  his  experi- 
ments are  given  here  in  considerable  detail. 

Employ  in  this  determination  the  apparatus  described  on 
p.  72  in  estimating  copper. 

Use  the  following  solutions : 

1.  A  fifth-normal  solution  of  sodium   carbonate.     This 
solution  constitutes  the  basis  of  value  of  the  subsequent  solu- 
tions. 

2.  A  dilute  solution  of  sulphuric  acid,  containing  about 
20  cubic  centimeters  of  acid  of  specific  gravity  1.84  in  4  liters 
of  water.     Standardize  this  on  the  sodium  carbonate  solu- 
tion. 

3.  A  dilute  ammonia  solution,  containing  about  50  cubic 
centimeters  of  ammonium  hydroxide  of  specific  gravity  0.95 
in  4  liters  of  water.     This    is  about  equivalent  in  strength 
to  the  standard  acid  solution.     Obtain  its  exact  ratio  by 
titration. 

4.  A  solution  of  copper  sulphate,   containing  about  80 
grams  of  CuSO4.5H2O  in  2  liters. 

Six  electrolytic  determinations  of  the  value  of  this  solu- 
tion were  made,  using  the  conditions :  25  cubic  centimeters 
of  copper  solution,  25  cubic  centimeters  of  standard  acid, 


DETERMINATION    OF    NITRIC    ACID. 

125  cubic  centimeters  dilution,  5  amperes,  10  volts,  ten 
minutes,  resulting  in  the  following  as  the  copper  content  of 
25  cubic  centimeters  of  the  sulphate  solution : 

GRAM.  GRAM. 

0.2532  0.2530 

0.2532  0.2536 

0.2535  0.2534 

The  average  of  these  values,  or  0.2533  gram,  was  used. 

The  acid  solution  and  the  ammonium  hydroxide  solution 
were  now  compared  with  each  other  and  with  the  sodium 
carbonate  solution,  litmus  or  methyl  orange  being  used  as 
indicators.  The  average  of  eight  concordant  results  is  as 
follows : 

Ten  cubic  centimeters  N/5  sodium  carbonate  =  10.22 
cubic  centimeters,  sulphuric  acid  =  9.960  cubic  centimeters 
of  ammonium  hydroxide  solution.  As  much  as  50  cubic 
centimeters  were  sometimes  consumed  in  one  titration  and  it 
is  believed  that  the  results  are  correct  for  three  figures  at 
least. 

An  additional  independent  standardization  of  the  ammon- 
ium hydroxide  solution  was  made  by  titrating  the  sulphuric 
acid  liberated  by  the  electrolysis  of  25  c.c.  of  the  copper 
solution  in  the  presence  of  25  cubic  centimeters  of  standard 
acid.  In  the  average  of  four  concordant  determinations  the 
total  free  acid,  after  electrolysis,  was  found  to  be  exactly 
neutralized  by  64.42  cubic  centimeters  of  the  ammonium 
hydroxide  solution;  deducting  the  24.38  cubic  centimeters, 
which  are  equivalent  to  the  25  cubic  centimeters  of  standard 
acid  present,  there  remain  40.04  cubic  centimeters  of  am- 
monium hydroxide  used  in  neutralizing  the  sulphate,  com- 
bined with  0.2533  gram  of  copper.  This  gives  a  ratio  of 
N/5  sodium  carbonate  to  the  ammonium  hydroxide  solution 
of  10 :  9.958,  agreeing  well  with  that  obtained  by  direct  titra- 
tion. 


292  ELECTRO-ANALYSIS. 

Experimental  Part. 

Weigh  off  the  desired  quantity  of  potassium  nitrate  and 
dissolve  it  in  a  small  amount  of  water  in  a  clean  platinum 
dish;  then  pipette  from  the  stock  solution  the  necessary 
amount  of  copper  sulphate  and  add  a  measured  amount  of 
standard  acid,  sufficient  to  make  the  electrical  resistance  low 
and  to  insure  the  solution  remaining  quite  strongly  acid  dur- 
ing the  reduction  of  the  nitrate. 

Dilute  to  about  125  cubic  centimeters  and  electrolyze  with 
about  4  to  5  a'mperes  and  about  10  volts.  The  exact  condi- 
tions are  stated  in  a  number  of  experiments  in  the  appended 
tabular  exhibit. 

During  the  course  of  the  electrolysis  the  copper  is  de- 
posited on  the  cathode  and  its  equivalent  of  sulphuric  acid 
is  liberated  and  added  to  the  acid  already  present,  whereby 
the  conductivity  is  increased  and  the  pressure  falls.  As  the 
nitric  acid  is  gradually  reduced  to  ammonia  the  free  acid 
becomes  neutralized  and  if  the  current  be  maintained  con- 
stant by  the  rheostat  the  pressure  will  gradually  rise  for 
about  twenty-eight  minutes  and  then  become  stationary, 
thereby  indicating  the  end  of  the  reduction.  This  rise  is 
usually  from  5  to  7  volts,  and  the  voltages  given  in  the  table 
are  those  read  at  the  outset  of  each  experiment,  to  which  the 
above  is  to  be  added  to  obtain  the  final  voltage. 

Stop  the  motor,  siphon  off  the  liquid  in  the  dish  into  a 
beaker  and  replace  it  by  distilled  watef  while  the  current 
passes;  the  dish,  anode  and  cover  glasses  are  well  washed, 
the  electrical  current  interrupted,  and  the  washings  added 
to  the  liquid  in  the  beaker.  It  is  unnecessary  to  weigh  the 
deposited  copper,  so  the  platinum  dish  is  merely  rinsed  with 
nitric  acid  and  washed  under  the  faucet,  when  it  is  ready  for 
use  again. 

Rapidly  neutralize  the  contents  of  the  beaker,  in  the  pres- 


DETERMINATION    OF    NITRIC    ACID.  293 

ence  of  litmus  or  methyl  orange  by  the  standard  ammonia 
solution  from  a  burette.  The  indicators  named  were  found 
to  give  identical  results.  Note  that  in  the  reaetion  of  reduc- 
tion one  molecule  of  potassium  nitrate  gives  rise  to  a  mole- 
cule of  potassium  hydroxide  and  one  of  ammonia;  hence 
two  equivalents  of  alkali  are  produced  from  one  equivalent 
of  nitrate,  and  allowance  must  be  made  for  this  by  having 
the  results  obtained  by  titration.  The  use  of  a  o. 5-gram 
sample  for  analysis  just  offsets  this.  The  calculation  of  the 
standard  ammonia  solution  to  its  equivalent  of  N/5  sodium 
carbonate  solution  and  thence  to  nitrogen  is  obvious. 

To  learn  the  best  conditions  a  number  of  experiments  may 
here  be  introduced  from  a  notebook. 

(a)  Time. — The  first  ten  experiments  were  made  with 
reference  to  the  time  of  reduction.     Using  25  cubic  centi- 
meters of  copper  sulphate  solution,  25  cubic  centimeters  of 
acid  solution  and  0.5  gram  of  nitrate,  5  amperes  gave  5.63 
per  cent.,  9.83  per  cent.,  9.91  per  cent.,  and  11.26  per  cent, 
of  nitrogen  respectively  in  ten,  fifteen  and  twenty  minutes, 
the  theoretical  percentage  of  nitrogen  in  potassium  nitrate 
being  13.86. 

Increasing  the  time,  with  4  amperes,  gave  13.64  per  cent, 
in  twenty-five  minutes  and  13.83  per  cent,  in  thirty  minutes. 

(b)  Amount  of  Copper  Sulphate. — The  above  results 
were  obtained  with  25  cubic  centimeters  of  copper  sulphate. 
Two  experiments  with  50  cubic  centimeters  gave  8.79  per 
cent,  in  twenty  minutes  and  12.96  per  cent,  in  thirty  min- 
utes, showing  that  the  increased  amount  of  copper  is  not  an 
advantage.     Two  experiments  with  but  15  cubic  centimeters 
of  copper  sulphate  solution  and  30  c.c.  of  standard  acid 
resulted  in  a  reduction  of  11.93  per  cent,  and  13.55  per  cent, 
in  twenty  and  thirty  minutes  respectively.     Increasing  the 
amount   of   acid   to   50  cubic  centimeters   with   the   same 


294  ELECTRO-ANALYSIS. 

amount  of  copper  gave  better  results,  viz.,  13.10  per  cent, 
and  13.83  per  cent,  in  twenty  and  thirty  minutes  respectively. 

(c)  Strength    of    Current.     An    experiment    with    5 
amperes  gave   13.38  per  cent,   of  nitrogen  in  twenty-five 
minutes,  while  6  amperes  gave  only  13.19  in  twenty  minutes. 
From  this  it  appears  that  4  amperes  is  sufficient  current, 
since  that  will  yield  complete  reduction  in  thirty  minutes  and 
more  current  will  not  do  the  work  in  less  time. 

(d)  Speed. — Two  experiments  with  the  speed  of  rota- 
tion of  the  anode  increased  to  about  560  revolutions  per 
minute  gave  12.91  per  cent,  and  13.19  per  cent,  in  twenty 
and  thirty  minutes  respectively;  the  voltage  needed  was  40, 
since  the  contact  between  the  anode  and  the  liquid  was  poor 
at   this   velocity.     So   much   heat   was   produced   that  .the 
liquid  boiled  freely,  but  no  advantage  in  increased  speed 

was  found. 

? 

The  results  and  detailed  conditions  of  this  work  are  found 
in  the  subjoined  tabular  exhibit.  They  indicate  that  the  con- 
ditions of  Experiment  8  are  to  be  preferred.  To  confirm 
this  a  series  of  ten  determinations  was  made  in  accordance 
with  these  conditions,  namely,  25  cubic  centimeters  of  cop- 
per sulphate  solution,  representing  0.2533  gram  of  metallic 
copper,  25  cubic  centimeters  of  the  standard  sulphuric  acid, 
0.5  gram  of  potassium  nitrate,  4  amperes,  10  volts  at  the 
outset,  or  17  volts  at  the  end  of  reduction,  slowest  speed  and 
thirty  minutes.  The  dish  was  not  warmed  at  the  outset  of 
the  experiment,  nor  was  external  heat  applied  during  elec- 
trolysis, although  the  liquid  was  considerably  warmed  by 
the  current,  the  final  temperature  being  about  65°  C.  This 
continuous  series  was  made  in  a  single  afternoon  and  no 
results  were  rejected;  consequently  the  latter  may  be  taken 
to  represent  the  probable  error  of  the  method. 


DETERMINATION    OF    NITRIC    ACID. 


295 


The  following  are  the  percentages  of  nitrogen  found,  the 
theoretical  value  being  13.86: 


PER  CENT. 
13.81 
13-79 
13-83 
13-83 
13-94 


PER  CENT. 
13-86 
13-92 
13.92 
13-86 
13-89 


Mean  of  the  series  of  ten,  13.865. 


TAKEN. 

CONDITIONS 

CALCULATION. 

o 

K 

H 

H 

** 

H 

c/i 

y 

a 

Q 
<!  <  H 

Q 
W 

o  o 

"  w 

H        §  (/i 

*P 

3 

£  " 

t>    "Z. 

a 

u 

< 
|| 

| 

i 

[INUTI 

Q  g  < 
lit 

§£* 

a  u  a 

H  Jj 
<  H< 

az   z 
0  0 

g  H  Q  5 

h  z 
O  & 

x 

COPPER  Si 
SOLUTIOJ 

Ij 
fe 

STANDARD 

X    £ 

£0 

1 

o 

a 

s 

« 

S 

H 

(3***  ° 

u 

AMMONIA 
EQUIV.' 

TO  CO 

AMMC 
EQUIVAL 
STANDAR 

g  ^  H 

U  <       u 

u    >    J    W 

i|3d 

PERCENTA 

GKN 

NUMBER  o 

25 

0.5000 

25 

0-2533 

12 

5 

IO 

44-5 

4O.O 

24.4 

19.9 

20.  i 

5.63 

I 

25 

0.5000 

25 

0-2533 

12 

5 

15 

29-5 

40.0 

24.4 

34.9 

35.1 

9.83 

2 

25 

0.5000 

25 

0-2533 

12 

5 

15 

29.2 

40.0 

24.4 

35-2 

35-4 

9.91 

3 

25 

0.5000 

25 

0-2533 

12 

5 

2O 

24-4 

40.0 

24.4 

40.0 

40.2 

11.26 

4 

25 

0.5000 

25 

0-2533 

8 

3 

2O 

32.4 

40.0 

24.4 

32.0 

32.2 

9.O2 

5 

25 

0.5000 

25 

0-2533 

IO 

4 

2O 

15.9 

40.0 

24.4 

48.5     48.7 

13.64 

6 

25 

0.5000 

25 

0.2533 

IO 

4 

25 

15.9 

40.0 

24.4 

48.5 

48.7 

13.64 

7 

25 

0.5000 

25 

0-2533 

9 

4 

30 

15.2 

40.0 

24-4 

49-2 

49  4 

13.83 

8 

25 

0.5000 

25 

0-2533 

9 

4 

30 

15-4 

40.0 

24.4 

49.0 

49-2 

13.78 

o 

25       0.5000 

25 

0.2533 

9 

4 

30 

15.5 

4O.O 

24.4 

48.9 

49-1 

13-75  10 

50      0.5000 

25 

0.5066 

10 

4 

20 

73-2 

80.0 

24.4 

31-2 

3L4 

8-79,11 

50      0.5000 

25 

0.5066 

10 

4 

30 

58.3 

80.0 

24.4 

46.  i 

46-3 

12.9612 

15 

o  5000 

3° 

0.1520 

10 

4 

20 

10.9 

24.O 

29-3 

42.4 

42.6 

II  93113 

15 

0.5000 

30 

o.  1520 

10 

4 

30 

5*  i 

24.O 

29.3 

48.2 

48.4 

13.55  H 

15 

0.5000 

50 

o.  1520 

10 

4 

20 

26.2 

24.0 

48.8 

46.6 

46.8 

13.1015 

15 

o.  5000 

50 

0.1520 

IO 

4 

30 

23.6 

24.0 

48.8 

49-2 

49-4 

13.83  16 

15 

0.5000 

50 

0.1520 

16 

6 

2O 

25-9 

24.0 

48.8 

46.9 

47.1 

13.19  17 

15 

0.5000 

0.1520 

12 

5 

25 

25-2 

24.O 

48.8 

47  6 

47-8 

13.3818 

25 

0.5000 

25 

0.2533 

40 

4 

2O 

18.5 

4O.O 

24-4 

45-9 

46.1 

12.91  19 

25 

0.5000 

25 

0.2533 

40 

4 

30 

17-5 

4O.O 

24.4 

46.9 

47-i 

13.19  20 

This  method  for  the  determination  of  nitrates  compares 
quite  favorably  with  other  methods  in  point  of  accuracy. 
Its  advantages  in  simplicity  and  speed  are  worthy  of  care- 


296  ELECTRO-ANALYSIS. 

ful  consideration,  as  a  complete  determination  of  the  nitric 
acid  content  of  an  alkali  nitrate  may  be  made  in  thirty-five 
minutes  from  the  time  of  weighing  off  the  sample. 

Recent  experiments,  made  in  this  laboratory,  have  dem- 
onstrated that  to  determine  the  nitric  acid  content  of  such 
salts  as  zinc  nitrate,  cobalt  nitrate,  nickel  nitrate,  etc.,  it 
is  advisable  to  precipitate  the  metal  with  sodium  carbonate, 
filter  out  the  precipitate  and  electrolyze  the  filtrate  contain- 
ing the  sodium  nitrate. 


6.  SPECIAL  APPLICATION  OF  THE  ROTAT- 
ING ANODE  AND  MERCURY  CATHODE 
IN  ANALYSIS. 

Determination  of  both  Cations  and  Anions. 

In  the  preceding  pages  numerous  examples  have  been 
given  of  the  determination  of  metals  with  the  help  of  the 
simple  device  pictured  (Fig.  17)  on  p.  58.  Under  copper, 
for  instance,  it  is  suggested  that  the  student  perform  the 
analysis  of  copper  sulphate,  depositing  the  metal  in  the 
mercury,  then  siphoning  off  the  colorless  solution  into  a 
beaker  and  determining  the  acid  by  titration  with  a  N/io 
solution  of  sodium  carbonate.  To  this  it  may  be  added 
that  no  more  satisfactory  method  can  be  adopted  in  the 
analysis  of  zinc  sulphate.  Both  constituents  can  be  rapidly 
and  accurately  estimated.  In  the  ordinary  gravimetric 
determination  of  the  sulphuric  acid  content  of  white  vitriol 
the  precipitate  of  barium  sulphate  is  very  apt  to  contain 
zinc,  so  by  this  electrolytic  procedure  the  analyst  gains  great 
advantage.  The  simplicity  of  the  procedure  appeals  strongly 
to  those  who  are  called  upon  to  perform  analyses  of  salts 


DETERMINATION    OF    CATIONS    AND    ANIONS.  297 

like  those  just  mentioned.  Indeed,  any  soluble  metallic 
sulphate  may  be  analyzed  in  this  manner.  The  results  have 
been  most  satisfactory.  When  the  method  was  first  applied 
to  them,  the  anode  was  stationary  (J.  Am.  Cherri.  S.,  25, 
883);  subsequently  it  was  rotated  (p.  58)  (J.  Am.  Chem. 
Soc.,  26,  1614;  Am.  Phil.  Soc.,  Pr.  XLIV,  137  (1905); 
J.  Am.  Chem.  S.,  27,  1527;  Myers,  J.  Am.  Ch.  S.,  26, 
1124.). 

Having  reached  a  high  degree  of  success  in  the  analysis 
of  sulphates  in  the  direction  outlined  in  the  preceding  para- 
graphs, it  occurred  to  the  writer  that  possibly  chlorides 
might  be  analyzed  equally  well  in  this  way  if  provision  were 
made  to  catch  or  fix  the  chlorine  ions.  Accordingly,  a 
solution  of  sodium  chloride  was  subjected  to  decomposition 
in  the  little  cup  (Fig.  17,  p.  58).  The  anode  consisted 
of  a  silver-plated  strip  of  platinum,  which  later  was  replaced 
by  a  weighed,  silver-coated  platinum  gauze  suspended  in 
the  aqueous  solution  (40  c.c.)  of  the  sodium  chloride. 
Almost  immediately  the  silver,  on  passage  of  the  current, 
began  to  darken  in  color  from  the  lower  edge  of  the  gauze 
upwards.  When  this  ceased,  the  decomposition  was  as- 
sumed to  be  at  an  end,  whereupon  the  gauze  was  raised 
from  the  solution,  rinsed  with  water  and  further  washed 
with  alcohol  and  ether.  It  was  weighed  after  drying  for 
a  short  time.  For  the  gauze  a  platinum  spiral  was  sub- 
stituted in  the  residual  liquor  in  the  beaker ;  the  current  was 
reversed,  the  layer  of  mercury  being  made  the  anode,  when 
the  sodium  was  rapidly  driven  into  the  water.  All  this 
occupied  about  twenty  minutes,  after  which  the  alkaline 
liquor  was  titrated  with  standardized  acid. 

A  solution  of  salt,  containing  0.0606  gram  of  chlorine 
and  0.390  gram  of  sodium  gave: 


298  ELECTRO-ANALYSIS. 

No.  C  GRAM.  Na  GRAM 

I 0.0606  0.0389 

2 0.0610  0.0384 

Six  hours  were  allowed  for  the  decomposition.  The  cur- 
rent showed  0.0325  to  0.03  ampere  and  2  volts. 

On  electrolyzing  a  solution  of  barium  chloride,  in  the 
same  way,  there  were  obtained : 

Ba  Cl                                                                Ba  Cl 

PER  CENT.  PER  CENT.                                           PER  CENT.  PER  CENT. 

55.87  28.69  instead  of               56.14  29.09 

56.07  29.31 

Strontium  bromide  was  analyzed  with  just  as  much  suc- 
cess. The  same  is  true  of  other  halides.  Indeed,  both 
sodium  chloride  and  barium  chloride  were  electrolyzed  suc- 
cessfully without  the  use  of  the  mercury  cathode.  A  flat, 
platinum  spiral  was  made  to  take  its  place.  The  alkaline 
liquors,  observing  proper  current  conditions,  did  not  inter- 
fere with  the  deposition  of  the  halogen  upon  the  silver  gauze. 

In  the  preceding  example  the  time  factor  was  somewhat 
prolonged  and  difficulty  was  experienced  in  determining  the 
end  of  the  reaction.  Hildebrand,  working  in  this  labora- 
tory, found  that  in  spite  of  the  extreme  care  in  keeping  the 
mercury  and  the  interior  of  the  cell  absolutely  clean  so  as 
to  minimize  secondary  decomposition  of  the  amalgam  some 
caustic  was  formed  and  after  the  halide  had  been  completely 
decomposed  it  was  possible  to  increase  the  weight  of  the 
gauze  indefinitely  by  the  production  of  silver  oxide  from  the 
electrolysis  of  the  caustic.  To  learn  the  end  of  the  decompo- 
sition the  following  scheme  was  pursued :  the  gauze  was 
suspended,  at  the  beginning  of  the  operation,  within  about 
5  mm.  of  the  surface  of  the  mercury  and  the  liquid  so 
diluted  as  to  cover  only  about  one-third  of  the  gauze.  The 
pressure  (voltage)  was  kept  constant  during  the  electrolysis 


DETERMINATION    OF    CATIONS    AND    ANIONS. 


299 


so  that  the  fall  in  current  strength,  as  the  action  progressed, 
indicated  the  completeness  of  the  decomposition.  When 
it  reached  from  0.005  to  0.02  amperes,  the  liquid  level  was 
raised  a  few  millimeters  from  time  to  time,  and  as  soon 
as  the  fresh  surface  showed  the  formation  of  brown  silver 
oxide — which  could  easily  be  distinguished  from  the  bluish 
chloride — the  gauze  was  removed,  immersed  in  alcohol, 
then  in  ether,  dried  and  weighed.  This  procedure  gave  con- 
secutive concordant  results.  In  every  case  the  amalgam 
was  washed  into  a  beaker  and,  after  it  had  decomposed,  the 
alkali  was  titrated  with  tenth  normal  sulphuric  acid,  using 
methyl  orange  as  an  indicator. 

Analysis  of  Sodium  Chloride. 

The  following  table  shows  the  results  obtained  for  this 
salt.  The  current  in  amperes,  at  the  beginning  and  end  of 
each  decomposition,  is  given  in  the  third  column. 


SODIUM  IN  GRAMS. 

CHLORINE  IN  GRAMS. 

TIME. 

Vni  T^ 

. 

MINUTES. 

,  V  OL  T  S. 

AMPERES. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

J35 

3-5 

.o8-.oi 

0.0460 

0.0461 

0.0708 

0.0713 

2IO 

3-5 

.09-.  003 

0.0460 

0.0456 

0.0708 

o  0706 

1S° 

3-5 

.20-.  005 

0.0460 

0.0460 

0.0708 

o.o;o6 

22O 

3-5 

.24-.  005 

0  0460 

o  0458 

0.0708 

00705 

2OO 

3-5 

.21-.  005 

0.0460 

0.0462 

0.0708 

0.0709 

1  2O 

3-5 

.i6-.oi 

o.  0460 

0.0459 

0.0708 

0.0712 

130 

3-5 

.20-.  02 

0.0460 

0.0461 

0.0708 

0.0705 

70 

3-5 

.I5-.04 

0.0460 

0.0459 

0.0708 

0.0707 

&s 

.14-.  03 

0.0460 

0.0463 

0.0708 

0.0711 

3-5 

.I3-.02 

0.0460 

0.0463 

0.0708 

0.0710 

The  deposits  were  perfectly  adherent  in  character  unless 
the  silver  coating  was  too  thin.  No  attempt  was  made  to 
protect  it  from  the  light,  so  that  the  deposits  both  here  and 
with  other  substances  were  always  very  dark  colored;  in 


300 


ELECTRO-ANALYSIS. 


fact,  with  several  other  salts  if  the  silver  salt  was  formed 
so  rapidly  as  to  show  its  true  color  at  places,  it  was  often 
not  very  adherent. 

Analysis  of  Sodium  Bromide. 


SODIUM  IN  GRAMS. 

BROMINE  IN  GKAMS. 

TIME. 

VOLTS 

A 

MINUTES. 

AMPERES. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

60 

4-0-3-5 

.13-.  02 

.0232 

'  -0235 

.0804 

* 
.0794 

45 

4.0-3.5 

.IJH.OS 

.0232 

.0237 

.0804 

.0806 

50 

3-5 

.12-.  03 

.0232 

.0231 

.0804 

.0806 

100 

3.5 

.i3-.oi 

.0232 

.0237 

.0804 

.0812 

60 

3-5 

.I2-.05 

.0232 

.0238 

.0804 

.0804 

3-5 

.09 

.0232 

.0230      .0804 

.0805 

Analysis  of  Sodium  Iodide. 


TIME. 

MINUTES. 

VOLTS. 

AMPERES. 

SODIUM  IN  GRAMS. 

IODINE  IN  GRAMS. 

PRESENT. 

FOUND. 

PRESENT 

FOUND. 

70 
70 

45 

4  -3-5 

3.5 
3.5-3 

.10-.  02 
.05-.OI 
.10-.  02 

.0154 
.0154 
.0154 

.0156 
.0156 
.0154 

.0850 
.0850 
.0850 

.0850 
.0857 
.0845 

Analysis  of  Potassium  Sulphocyanide. 

This  salt  proved  more  troublesome  because  the  potassium 
amalgam  usually  started  to  decompose  rapidly  near  the 
end  of  the  electrolysis. 


POTASSIUM  IN  GRAMS 

CNS  IN  GRAMS. 

TIME. 

VOLTS 

. 

PERES. 

MINUTES. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

45 

3-5 

.IO-.O6 

•0375 

.0371 

.0558 

.0558 

120 

3-5 

.o7-.o4 

•0375 

•0379 

.0558 

.0560 

105 

4-3-5 

.IO-.OI 

•0375 

•0379 

.0558 

.0560 

135 

3-5 

.06-.  oi 

•0375 

•0375 

•0558 

.0566 

65 

4-3-5 

.09-.  oi 

•0375 

.0373 

.0558 

•0553 

DETERMINATION    OF    CATIONS    AND    ANIONS.  3OI 

It  was  soon  after  found  that  silver  ferro-  and  ferri- 
cyanides  could  be  formed  and,  what  seemed  still  more  re- 
markable, silver  carbonate.  In  the  last  instance  the  decom- 
position was  complete,  there  being  no  traces  of  carbon 
dioxide  liberated  at  the  anode.  The  deposit,  afterwards 
immersed  in  dilute  sulphuric  acid,  liberated  carbon  dioxide 
with  effervescence.  However,  it  was  impossible  to  make 
these  depositions  quantitative,  because  the  silver  salts  were 
not  very  coherent  and  at  the  edge  of  the  gauze  near  the 
mercury,  where  the  deposit  was  thick,  part  of  it  always 
became  detached. 

The  difficulty  here  mentioned  was  overcome  by  devising 
a  new  anode.  This  consisted  of  two  circular  disks  of  plati- 
num gauze  5  cm.  in  diameter  and  having  300  meshes  per 
square  centimeter.  The  circumference  was  slightly  fused 
in  the  blowpipe.  These  were  mounted  5  mm.  apart  on  a 
stout  platinum  wire  i  mm.  in  diameter  and  10  cm.  long 
which  passed  through  the  centers  of  the  disks  perpendicular 
to  them.  Each  disk  was  attached  to  this  axial  wire  by 
means  of  two  smaller  wires  fitting  tightly  into  two  adjacent 
holes  drilled  at  right  angles  to  each  other  through  the  large 
wire.  These  anodes  weighed  about  16  grams  apiece.  The 
total  surface  of  each  pair  of  disks  is  about  100  sq.  cm.  which 
is  at  least  doubled  when  coated  with  several  grams  of  silver. 
These  anodes  were  always  supported  when  not  in  use  by 
fastening  the  axis  in  a  clamp  so  that  the  gauze  might  not 
come  in  contact  with  anything  which  might  bend  it.  In 
order  to  suspend  them  from  the  balance  beam  in  weighing,  a 
loop  of  fine  platinum  wire  was  soldered  to  each  axial  wire 
about  2  cm.  from  the  top. 

Silver  Plating  the  Anode. — In  plating  the  anodes  with 
silver  the  rotator  was  always  used,  as  a  coating  from  3  to  4 
grams  of  silver  could  thus  be  deposited.  A  number  of  de- 


302  ELECTRO-ANALYSIS. 

terminations  could  then  be  made  without  replating  the  gauze, 
the  deposited  silver  chloride  being  merely  dissolved  off  by 
immersing  for  a  few  moments  in  potassium  cyanide,  thus 
exposing  a  fresh  surface  of  silver.  The  plating  was  done 
in  a  beaker,  the  anode  being  a  platinum  wire  passing 
through  a  glass  tube  to  the  bottom  of  the  beaker  where  it 
was  bent  into  a  flat  horizontal  spiral.  A  strong  stock  solu- 
tion of  silver  potassium  cyanide  was  kept  in  a  bottle  and 
portions  added  to  the  beaker  from  time  to  time  as  the  sil- 
ver in  the  electrolyte  was  deposited.  No  particular  care  is 
necessary  in  this  plating  as  the  conditions  may  be  varied 
rather  widely  without  injuring  the  deposit;  about  5  volts 
and  i  to  2  amperes  were  the  ordinary  conditions.  When 
the  coating  was  sufficiently  heavy  the  gauze  was  removed, 
washed  by  immersing  in  distilled  water,  followed  by  alco- 
hol and  ether. 

To  avoid  the  necessity  of  centering  the  anode  each  time 
it  was  placed  in  the  rotator,  a  small  piece  of  copper  foil  was 
rolled  into  a  cylinder  about  the  axis  of  the  anode  and  then 
put  permanently  into  the  tip  of  the  rotator.  The  anode  was 
thus  always  centered  when  put  in  position. 

The  Cell. — In  principle  it  resembles  the  Castner-Kellner 
process  for  caustic  soda,  the  amalgam  being  formed  in  one 
compartment  and  decomposed  in  another.  The  outer  cell 
is  a  crystallizing  dish  n  cm.  in  diameter  and  5  cm.  deep. 
Inside  of  this  is  a  beaker  6  cm.  in  diameter  with  the  bottom 
cut  off  and  the  edge  rounded  so  that  a  ring  is  formed  4.5  cm. 
high.  This  rests  on  a  large  Y  of  thin  glass  rod  on  the 
bottom  of  the  crystallizing  dish  and  is  kept  in  position  by 
three  rubber  stoppers  fitting  radially  between  it  and  the 
inside  of  the  dish.  In  the  outer  compartment  thus  formed 
there  is  a  ring  of  about  six  turns  of  nickel  wire  provided 
with  three  legs  which  are  fastened  to  the  ends  of  the  glass  Y 


DETERMINATION    OF    CATIONS    AND    ANIONS.  303 

and  serve  to  support  the  ring  about  i  cm.  above  the  surface 
of  the  mercury  when  sufficient  of  the  latter  is  poured  in 

FIG.  39- 


to  seal  off  the  two  compartments.     The  cell  and  anode  are 
shown  in  Fig.  39. 


3C4 


ELECTRO-ANALYSIS. 


In  using  this  cell,  which  must  be  kept  scrupulously  clean, 
pure  clean  mercury  is  poured  in  so  that  its  level  is  about  3 
mm.  above  the  lower  edge  of  the  bottomless  beaker.  The 
solution  to  be  electrolyzed  is  then  put  into  the  inner  com- 
partment; into  the  outer  is  placed  enough  distilled  water  to 
cover  the  nickel  wire,  and  to  this  is- added  a  cubic  centimeter 
of  a  saturated  solution  of  common  salt.  By  this  arrange- 
ment the  amalgam  formed  in  the  inner  compartment  is  im- 
mediately decomposed  in  the  outer,  which  acts  as  a  cell 
whose  elements  are  amalgam-sodium  chloride-nickel  wire. 
The  sodium  chloride  serves  merely  to  make  the  liquid  a  con- 
ductor so  that  the  action  may  proceed  more  rapidly  at  the 
beginning.  Without  this  scheme  the  amalgam  is  not  en- 
tirely decomposed  in  the  outer  compartment  as  pure  water 
does  not  attack  it  rapidly  enough  to  prevent  a  partial  decom- 
position in  the  inside  cell.  The  mercury  is  connected  with 
the  negative  pole  of  the  battery  by  means  of  the  glass  tube 
bearing  the  copper  and  platinum  wires  described  above, 
which  dips  into  the  outer  compartment.  After  the  electrol- 
ysis is  complete  the  entire  contents  of  the  cell  are  poured 
into  a  beaker,  the  cell  rinsed  and  the  alkali  titrated.  After 
titration  the  mercury  is  washed,  the  water  decanted  and  the 
metal  poured  into  a  large  separatory  funnel,  from  which  it 
can  be  drawn  off  clean  and  dry.  To  show  how  well  this 
new  arrangement  of  anode  and  new  cell  worked  in  the 
analysis  of  sodium  chloride  the  following  results  attest : 


TIME. 
MINUTES 

VOLTS. 

AMPERES. 

SODIUM  IN  GRAMS. 

CHLORINE  IN  GRAMS. 

PRESENT. 

FOUND. 

PRESENT 

FOUND. 

30 

4.0-2.5 

.50-02 

.0461 

•0459 

.0708 

.0704 

45 

3-5-2-5 

.34-01 

.0461 

.0708 

.0706 

40 

3  5-3-0 

.50-01 

.0461 



.0708 

.0704 

45 

4-0-3-5 

.65-01 

.0461 

— 

.0708 

.0716 

3° 

4.0-2.5 

.76-02 

.0461 

— 

.0708 

•0713 

55 

3-0 

.  26-O2 

.0461 

— 

.0708 

.0709 

DETERMINATION    OF    CATIONS    AND    ANIONS. 


305 


Thus  far  the  anode  has  remained  stationary.  Hence- 
forth, all  results  given  will  be  those  obtained  with  the  help 
of  the  rotating1  anode. 

The  weighed  gauze  anode  should  be  clamped 'to  the  shaft. 
Lower  the  latter  in  the  cell  till  the  lower  gauze  is  about  5 
mm.  from  the  surface  of  the  mercury.  Adjust  the  motor 
and  the  belt,  start  the  motor  and  turn  on  the  electrolyzing 
current.  The  most  convenient  speed  for  the  motor  would 
be  about  300  revolutions  per  minute. 

Do  not  wash  the  anode  after  the  salt  is  decomposed  as  the 
water  remaining  is  pure.  This  avoids  any  loss  by  the  usual 
washing  in  water,  alcohol  and  ether,  although  the  two  may 
be  used  where  it  is  desired  to  still  further  reduce  the  time. 
Dry  the  gauze  over  a  steam  radiator. 

Analysis  of  Sodium  Bromide. 

Let  the  dilution  of  the  salt  solution  be  about  25  cubic 
centimeters.  Only  the  lower  gauze  needs  to  be  immersed 
as  it  will  afford  surface  sufficient  for  the  quantity  of  bromide 
generally  used  in  experiments. 

RESULTS. 


TIME. 
MINUTES. 

VOLTS. 

AMPERES. 

SODIUM  IN  GRAMS. 

BROMINE  IN  GRAMS. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

30 
30 

5-° 

5-o 

.65-.OI 
.65-.OI 

0231 
.02JI 

•0233 
•0233 

.0800 
.0800 

.0798 
.0802 

Analysis  of  Sodium  Carbonate. 

In  this  determination  it  is  well  to  have  the  silver  anode 
surface  slightly  roughened.  This  can  be  obtained  by  stop- 
ping the  rotator  several  minutes  before  removing  the  gauze 
anode  from  the  silver  plating  bath. 

27 


306 


ELECTRO-ANALYSIS. 


RESULTS. 


SODIUM  IN  GRAMS. 

CO3  IN  GRAMS. 

MINUTES. 

VOLTS. 

AMPERES 

PRESENT. 

FOUND 

PRESENT. 

FOUND. 

60 

3-5-5-0 

.i5-.oi 

-0323 

.0325 

.0420 

.0416 

90 

4.0-5.0 

.  15-.  01 

•0323 

.0324 

.0420 

.0419 

50 

5-o 

.65-.oi 

.0346 

•0349 

.0450 

.0448 

70 

3-5-5-0 

.15-01 

.0346 

.0450 

.0447 

In  this  instance  the  easiest  way  to  clean  the  gauze  is  to 
ignite  it  gently  instead  of  the  usual  washing  with  potassium 
cyanide,  water  and  then  drying. 

Analysis  of  Potassium  Ferrocyanide. 


TIME. 
MINUTES. 

VOLTS. 

AMPERES. 

POTASSIUM  IN  GRAMS. 

Fe(CN)6  IN  GRAMS. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

30 

4.0-4.5 

.i5~.oi 

.0391 

.0384 

.0531 

•0531 

30 

3  0-5.0 

.i5-.oi 

.0391 

.0389 

•0531 

•0532 

30 

4.0-5.0 

.2O-.OI 

.0391 

.0387 

.0531 

.0527 

Analysis  of  Potassium  Ferricyanide. 


POTASSIUM  IN  GRAMS. 

Fe(CN)6  IN  GRAMS. 

TIME. 
MINUTES. 

VOLTS. 

AMPERES. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

35 

2  -5 

.20-.OI 

.0392 

.0710 

.0714 

30 

4  -5 

.40-.  oi 

.0392 

.0389 

.0710 

.0712 

40 

4-5-5 

.3o-.oi 

.0392 

.0389 

.0710 

.0713 

Analysis  of  Trisodium  Phosphate. 

Trisodium  phosphate  gave  a  deposit  which  was  satis- 
factory at  4  volts  but  not  completely  adherent  at  5  volts 
The  lower  voltage  and  the  smaller  conductivity  made  a 
longer  time  necessary  to  get  out  the  last  traces.  To  avoid 
this,  in  the  last  two  determinations  a  second  anode  was  used 
near  the  end  to  receive  these  traces. 


DETERMINATION    OF    CATIONS    AND    ANIONS. 


307 


SODIUM  IN  GRAMS. 

l'O4  IN  GRAMS. 

TIME. 
MINUTES. 

VOLTS. 

AMPERES. 

PRESENT. 

FOUND. 

PRESENT. 

FOUND. 

75 

5-4 

•50 

•0343 

•0343 

.0472 

•0473 

120 

4 

•30 

•0343 

•0343 

.0472 

.0468 

60 

4 

•30 

•0343 

.0340 

.0472 

.0470 

70 

4 

.40 

•0343 

— 

.0472     .0478 

See  Hiklebrancl,  J.  Am.  Ch.  S.,  29,  447. 


Finding  that  halides  of  the  alkali  metals  were  so  readily 
analyzed  in  the  manner  outlined,  it  was  but  a  step  to  the 
application  of  the  same  procedure  to  the  alkaline  earth 
metals.  The  appended  results  were  obtained,  in  this  labora- 
tory, by.  Hiram  S.  Lukens  and  Thos.  P.  McCutcheon,  Jr. 

Thus,  on  dissolving  a  definite  amount  of  barium  chloride 
in  water  and  electrolyzing  with  a  current  of  0.3  ampere 
and  2.5  to  3  volts,  it  was  discovered  that  as  much  as  0.2 
gram  of  metal  and  its  equivalent  of  halogen  could  be  readily 
determined  in  from  thirty  to  forty  minutes. 

EXAMPLES. 


BARIUM  PRESENT. 

BARIUM  FOUND. 

CHLORINE 
PRESENT. 

CHLORINE  FOUND 

0.2277  gram 

0.2276  gram 

0.1180  gram 

0.1177  gram 

0.2274 

0.1178 

0.2277 

o.  1181 

0.2278 

0.1180 

0.2277 

o.  1180 

0.2277 

0.1181 

The  bromide  was  used  in  the  determination  of  strontium. 
The  conditions  were  those  used  under  barium  chloride. 


308  ELECTRO-ANALYSIS. 

EXAMPLES. 

STRONTIUM  PRESENT.  STRONTIUM  FOUND. 

0.0727  gram  0.0725  gram 

0.0727  gram 
0.0727  gram 
0.0726  gram 
0.0725  gram 

The  barium  and  strontium  amalgams  passed  freely  into 
the  outer  dish  and  there  quickly  decomposed. 

Upon  electrolyzing  a  solution  of  pure  magnesium  chloride 
large  quantities  of  magnesium  hydrate  were  formed  in  the 
inner  dish  or  compartment,  while  not  a  trace  of  magnesium 
could  be  detected  in  the  outer  compartment. 

Mixtures  of  calcium  chloride  and  magnesium  chloride, 
consisting  of  one  half  as  much  magnesium  as  calciu'm  or  of 
equal  amounts,  gave  like  results.  Not  even  traces  of  calcium 
or  magnesium  were  found  in  the  outer  dish,  provided  the 
current  did  not  exceed  3.5  to  4  volts. 

Separation  of  Sodium  from  Calcium  and  Magnesium. 

As  the  amalgams  of  calcium  and  magnesium  decomposed 
so  easily,  it  was  thought  that  this  separation  could  be  made. 
Accordingly  the  chlorides  of  the  three  metals  were  dissolved 
in  water  and  the  solution  placed  in  the  inner  dish.  It  was 
then  exposed  for  a  period  of  fifty  minutes  to  the  action  of  a 
current  of  0.25  ampere  and  3.5  volts. 

Calcium    present    in    grams 0.0222 

Magnesium  present   in  grams 0.0210 

Sodium    present    in    grams 0.0474 

Sodium    found    in    grams 0.0471 

Sodium    found    in    grams 0.0474 

Sodium    found    in    grams 0.0476 

Sodium    found    in    grams 0.0474 


DETERMINATION    OF    CATIONS    AND    ANIONS. 


309 


Separation  of  Potassium  from  Calcium  and  Magnesium. 
Using  like  amounts  of  calcium  and  magnesium  in  the  form 
of  chlorides,  and  substituting  potassium  chloride  for  sodium 
chloride,  while  applying  the  same  current  as  in  the  preceding 
separation,  the  following  quantities  of  potassium  were  found 
in  the  outer  dish : 


GRAM. 
0.0582 
0.0583 
0.0580 


GRAM. 

0.0579 
0.0580 
0.0580 


The  quantity  of  potassium  present  equaled  0.0580  gram. 

Separation  of  Barium  from  Calcium  and  Magnesium. 

Dissolve  the  chlorides  in  30  cubic  centimeters  of  water, 
add  one  drop  of  hydrochloric  acid  (i  :  10)  to  this  solution 
and  electrolyze  with  a  current  of  0.3  ampere  and  3.5  to  4 
volts  for  a  period  of  seventy-five  minutes. 

EXAMPLES. 


BARIUM  PRESENT 
IN  GRAMS. 

CALCIUM  PRESENT 
IN  GRAMS 

MAGNESIUM 
PRESENT  IN 
GRAMS. 

BARIUM  FOUND 
IN  GRAMS. 

0.0455 

0.0222 

0.0210 

0.0456 

0.0455 

0.0454 

0.0454 

0.0455 

0.0454 

0.0454 

o.o 

910 

0.0910 

0.0911 

0.0910 

0.0912 

0.0910 

When  calcium  and  magnesium  are  present  together  as 
chlorides   their   electrolysis   leads   to   amalgam    formation. 


3IO  ELECTRO-ANALYSIS. 

These  amalgams,  however,  decompose  in  the  inner  cell, 
forming  hydroxides.  Under  such  conditions,  viz.,  the 
presence  of  magnesium  and  working  with  a  pressure  not 
exceeding  five  volts,  the  calcium  is  retained  within  the  inner 
cell.  The  separation  of  barium  from  calcium  and  mag- 
nesium was  thus  made  possible,  as  previously  outlined.  If, 
however,  calcium  chloride  be  subjected  to  a  higher  pressure 
(8  volts),  it  will  be  fully  decomposed,  the  chlorine  attach- 
ing itself  to  the  silver-plated  anode  and  the  metal  forming 
an  amalgam,  passing  into  the  outer  dish  or  compartment. 
Numerous  determinations  proved  this. 

Electrolysis  of  a  Mixture  of  Barium,  Calcium  and 
Magnesium  Chlorides. 

Let  the  solution  contain  0.0691  gram  of  barium,  0.0278 
gram  of  calcium  and  0.0220  gram  of  magnesium.  Electro- 
lyze  the  solution,  after  the  anode  has  begun  to  rotate,  with 
a  pressure  of  3.5  volts.  In  two  hours  the  barium  amalgam 
will  have  formed  and  completely  decomposed  to  hydrate, 
in  the  outer  compartment.  Titrate  this  hydrate,  then  in- 
crease the  pressure  to  9  volts,  the  current  ranging  from 
0.30  to  0.02  ampere.  In  three  hours  the  calcium  will  be 
completely  removed  to  the  outer  cell,  and  may  there  be 
titrated  with  tenth  normal  acid.  One  illustration  of  the 
results  from  a  solution,  constituted  as  above  indicated, 
showed  the  barium  found  to  be  0.0691  gram,  the  calcium 
0.0276  gram,  leaving  of  course  as  residuum  the  quantity 
of  magnesium  originally  added. 

Consult  also  Coehn  and  Kettembeil,  Z.  f.  anorg.  Chem., 

38,   198  tO  2T2. 

Separation  of  Strontium  from  Calcium  and  Magnesium. 

Use  the  conditions  given  in  the  separation  of  barium 
from  the  same  metals.  Results  like  the  following  were 
obtained. 


DETERMINATION    OF    CATIONS    AND    ANIONS.  3  I  I 

STRONTIUM  PRESENT  IN  GRAMS.  STRONTIUM  FOUND  IN  GRAMS. 

0-0565  0.0563 

0-0565  0.0565 

0.0565  0.0564 

0-0565  0.0565 

0.0565  0.0566 

0-0565  0.0565 

Barium  from  Magnesium. 

Use  the  chlorides  in  water  solution.  Let  the  current 
equal  0.3  ampere  and  3.5  volts.  The  anode  should  per- 
form 300  revolutions  per  minute.  The  current  will  not 
fall  below  0.03  ampere,  due  to  the  traces  of  magnesium 
hydrate  which  have  passed  into  solution.  Several  results 
show  the  accuracy  of  the  method. 


BARIUM  PRESENT 

MAGNESIUM  PRESENT 

I>ARIUM    FOUND 

IN  GRAMS. 

IN  GRAMS. 

IN  GRAMS. 

0.0455 

0.0358 

0-0455 

0.0455 

0.0358 

0.0456 

0.2277 

0.0358 

0.2277 

0.2277 

0.0358 

0.2277 

Strontium  from  Magnesium. 

Use  the  same  conditions  as  were  employed  in  the  pre- 
ceding separation. 

Barium  from  Iron. 

Electrolyze  the  solution  of  the  chlorides  as  neutral  as 
possible  with  a  current  of  0.3  ampere  and  3  to  5  volts  for 
a  period  of  fifty  minutes.  The  iron  amalgam  decomposes 
at  once  within  the  inner  compartment,  forming  ferric  hy- 
drate, while  the  barium  amalgam  passes  into  the  outer  cup 
and  rapidly  decomposes  there.  The  results  were  most 
satisfactory. 

Strontium,  Potassium  and  Sodium  may  be  similarly 
separated  from  Iron.  The  results  in  all  instances  were 
excellent. 


3  I  2  ELECTRO-ANALYSIS. 

Barium,  Strontium,  Potassium  and  Sodium  were,  with 
conditions  like  those  given  under  barium  from  iron,  sepa- 
rated most  satisfactorily  from  Aluminium. 

Sodium  from  Uranium. 

Use  the  chlorides,  apply  a  current  of  3.5  volts  and  0.3 
to  0.02  ampere.  The  time  is  usually  three  hours.  The 
chlorine  collects  on  the  silver-plated  anode.  The  inner 
compartment  will  be  filled  with  yellow  colored  uranium 
hydroxide  which  gradually  assumes  a  black  color.  The 
sodium  hydroxide,  formed  in  the  outer  dish  or  compart- 
ment, should  be  titrated  with  tenth  normal  hydrochloric 
or  sulphuric  acid,  using  methyl  orange  as  an  indicator. 
Sometimes  it  is  more  convenient  to  remove  the  anode  when 
the  decomposition  is  finished,  siphon  out  the  liquid  and  the 
hydroxide  formed  there,  wash  out  the  inner  compartment 
thoroughly  with  pure  water,  then  pour  the  contents  of  the 
cell  into  a  large  beaker,  and  there  make  the  titration  with- 
out the  slightest  difficulty. 

Potassium  and  lithium  may  be  separated,  under  like 
conditions,  from  uranium.  When  making  the  separation 
of  lithium  use  a  current  of  0.3  to  o.oi  ampere  and  5  volts. 
Time  one  hour. 

Barium  from  Uranium. 

This  separation  may  be  made  in  one  hour  by  employing 
a  current  of  1.5  to  o.oi  amperes  and  5  volts.  It  is  well 
to  acid  a  definite  volume  of  tenth  normal  hydrochloric  acid 
to  the  water  in  the  outer  dish.  Any  barium  hydroxide  or 
carbonate  that  might  form  there  is  at  once  dissolved  and 
at  the  conclusion  of  the  experiment  it  is  only  necessary  to 
titrate  the  residual  acid. 

In  separating  strontium  from  uranium  follow  the  pre- 


DETERMINATION    OF    CATIONS    AND    ANIONS.  313 

ceding  plan  and  use  a  current  of  0.4  to  0.02  ampere  and  5 
volts.     Two  hours  will  suffice  for  the  separation. 

With  a  current  varying  from  0.4  to  o.oi  ampere  and  a 
pressure  of  4  to  5  volts,  it  is  possible,  using  chlorides,  to 
separate  barium  completely,  in  a  period  of  two  hours,  from 
cerium,  lanthanum,  neodymium,  thorium  and  titanium. 
The  amalgams  of  the  rare  earth  metals  form  hydroxides  at 
once  -in  the  inner  cell,  while  the  barium  amalgam,  passing 
into  the  outer  compartment,  there  decomposes.  Consult 
also  Kettembeil,  Z.  f.  anorg.  Ch.,  38,  213. 

The  Analysis  of  Sodium  Sulphide. 

Coat  the  platinum  disks  with  cadmium,  then  carefully 
dry,  weigh  and  suspend  them  in  the  aqueous  solution  of  a 
known  amount  of  sodium  sulphide.  Use  a  current  of  o.i 
to  0.03  ampere  and  3.5  volts.  In  fifteen  minutes  the  an- 
alysis will  have  been  completed.  At  first  the  solution  in 
the  inner  cup  will  assume  a  yellow  color.  After  a  few 
minutes,  however,  it  will  be  colorless.  In  a  sample  con- 
taining 0.0253  gram  of  sulphur  there  was  found : 

0.0252  gram  of  sulphur 
0.0252  gram  of  sulphur 
0.0251  gram  of  sulphur 

The  deposit  of  cadmium  sulphide  is  very  adherent.  It 
should  be  dried  at  about  115°  C.,  before  weighing. 

In  the  analysis  of  alkaline  fluorides  the  anode  disks  may 
be  coated  with  calcium  hydrate.  On  electrolyzing  sodium 
fluoride  the  halogen  will  attach  itself  to  the  calcium  hy- 
drate on  the  anode,  forming  there  an  adherent  layer  of 
calcium  fluoride.  The  alkali  metal  will  pass  out  into  the 
larger  compartment  of  the  cell,  decomposing  to  hydroxide 
and  be  there  titrated.  Numerous  decompositions  have 
28 


314  ELECTRO-ANALYSIS. 

been  successfully  made  in  this  laboratory,  but  as  the  study 
is  still  in  progress,  this  mere  mention  will  be  here  made. 


7.  OXIDATIONS  BY  MEANS  OF  THE 
ELECTRIC  CURRENT. 

LITERATURE. — Smith,  Ber.,  23,  2276;  Am.  Ch.  Jr.,  13,  414;  Frankel, 
Ch.  N.,  65,  64. 

When  natural  sulphides,  e.  g.,  chalcopyrite,  marcasite, 
etc.,  are  exposed  to  the  action  of  a  strong  current  in  the 
presence  of  a  sufficient  quantity  of  potassium  hydroxide, 
their  sulphur  will  be  quickly  and  fully  oxidized  to  sul- 
phuric acid  (Jr.  Fr.  Ins.,  April,  1889;  Ber.,  22,  1019). 
The  metals  (iron,  copper,  etc.)  originally  present  in  the 
mineral  separate  as  oxides  and  metal  on  dissolving  the 
fused  alkaline  mass  in  water.  This  method  of  oxidation 
eliminates  many  other  disagreeable  features  of  the  old 
methods.  Its  rapidity  and  accuracy  entitle  it  to  the  fol- 
lowing brief  description  :— 

Place  about  20  grams  of  caustic  potash  in  a  nickel 
crucible  ii  inches  high  and  if  inches  wide.  Apply  heat 
from  a  Bunsen  burner  until  the  water  has  been  almost  en- 
tirely expelled,  when  the  flame  is  lowered  so  that  the  tem- 
perature is  just  sufficient  to  retain  the  alkali  in  a  liquid 
condition.  The  crucible  is  next  connected  with  the  nega- 
tive pole  of  a  battery,  and  the  sulphide  to  be  oxidized  is 
placed  upon  the  fused  alkali.  As  some  natural  sulphides 
part  with  a  portion  of  their  sulphur  at  a  comparatively 
low  temperature,  it  is  advisable  to  allow  the  alkali  to  cool 
so  far  that  a  scum  forms  over  its  surface  before  adding  the 
weighed  mineral. 

The  heavy  platinum  wire,   attached  to  the  anode,   ex- 


OXIDATIONS    BY    MEANS    OF    ELECTRIC    CURRENT.       315 

tends  a  short  distance  below  the  surface  of  the  fused  mass. 
When  the  current  passes,  a  lively  action  ensues,  accom- 
panied with  some  spattering.  To  prevent  loss  from  this 
source,  always  place  a  perforated  watch  crystal  over  the 
crucible.  After  the  current  has  acted  for  10-20  minutes, 
interrupt  it.  When  the  crucible  and  its  contents  are  cold, 
place  them  in  about  200  c.c.  of  water,  to  dissolve  out  the 
excess  of  alkali  and  alkaline  sulphate.  Filter.  Invaria- 
bly examine  the  residue  for  sulphur  by  dissolving  it  in 
nitric  acid  and  then  testing  with  barium  chloride.  The 
alkaline  filtrate  is  carefully  acidulated  with  hydrochloric 
acid,  and  after  digesting  for  some  time  is  precipitated  with 
a  boiling  solution  of  barium  chloride.  When  the  hydro- 
chloric acid  is  first  added,  care  should  be  taken  to  observe 
.whether  hydrogen  sulphide  or  sulphur  dioxide  is  liberated. 
If  the  oxidation  is  incomplete  sulphur  also  makes  its  ap- 
pearance as  a  white  turbidity.  The  caustic  potash  em- 
ployed in  these  oxidations  should  always  be  examined  for 
sulphur  and  other  impurities.  As  it  is  difficult  to  obtain 
alkali  perfectly  free  from  sulphur  compounds,  a  weighed 
portion  should  be  taken  and  its  quantity  of  sulphur  de- 
ducted from  that  actually  found  in  the  analysis. 

The  arrangement  of  apparatus  employed  in  the  oxida- 
tions just  outlined  is  represented  in  Fig.  40.  The  crucible  A 
is  supported  by  a  stout  copper  wire  bent  as  indicated,  and 
held  in  position  by  a  binding  screw  attached  to  the  base  of  a 
filter  stand.  The  arm  of  the  latter  carries  a  second  bind- 
ing screw  holding  the  platinum  anode  in  position.  While 
the  platinum  rod  is  generally  the  positive  electrode,  it  is 
best  to  make  it  the  negative  pole  for  at  least  a  part  of  the 
time  during  which  the  current  acts.  This  is  advisable 
because  in  many  of  the  decompositions  metals  are  pre- 
cipitated upon  the  sides  of  the  crucibles,  and  can  readily 


316 


ELECTRO-ANALYSIS. 


OXIDATIONS    BY    MEANS    OF    ELECTRIC    CURRENT.       3  I/ 

enclose  unattacked  sulphide,  so  that  by  reversing  the 
current  (the  poles)  any  precipitated  metal  will  be  detached, 
and  the  enclosed  sulphide  be  again  brought  into  the  field 
of  oxidation.  Cinnabar  is  a  sulphide  which  has  a  tendency 
to  mass  together,  and  it  could  only  be  decomposed  and  its 
sulphur  thoroughly  oxidized  by  reversing  the  current  every 
few  minutes.  To  reverse  the  current  use  the  contrivance 
C ;  this  is  nothing  more  than  a  square  block  of  wood  fastened 
to  the  top  of  the  table,  T,  by  a  screw  or  nail.  The  four 
depressions  (.v)  in  it  contain  a  few  drops  of  mercury,  into 
which  the  side  binding  screws  (a)  project.  The  mercury 
cups  are  made  to  communicate  with  each  other  by  a  cap  of 
wood,  D,  carrying  two  wires,  which  pass  through  it  and 
project  a  slight  distance  on  its  lower  side.  By  raising  the 
cap  and  turning  it  so  that  the  wires  are  vertical  (  *  )  or 
horizontal  ( >),  the  crucible  or  the  platinum  wire  extend- 
ing into  the  fused  mass  can  be  made  the  anode  or  cathode 
in  a  few  seconds.  £  is  a  Kohlrausch  amperemeter  and  R 
the  resistance  frame  (Fig.  6). 

Storage  batteries  furnish  the  most  satisfactory  current 
for  work  of  this  character.  In  the  sketch  the  cells  stand 
beneath  the  table;  the  wire  from  the  anode  passes  through 
a  hole  in  the  table- top,  and  is  attached  to  one  of  the  bind- 
ing-posts of  the  block  C,  while  the  positive  wire  is  attached 
to  a  binding-post  at  the  end  of  the  table-top,  and  from 
here  it  passes  to  the  resistance  frame,  R,  where  it  is  fixed 
by  an  ordinary  metallic  clamp. 

For  most  purposes  the  strength  of  current  need  not 
exceed  1—1.5  amperes;  however,  it  may  be  necessary 
occasionally  to  increase  it  to  4  amperes.  Pyrite,  FeS2,  is 
even  then  not  completely  decomposed.  This  particular 
case  requires  the  addition  of  a  quantity  of  cupric  oxide 
equal  in  weight  to  the  pyrite  and  a  current  of  the  strength 


3  1 8  ELECTRO-ANALYSIS. 

last  indicated  before  all  of  its  sulphur  is  fully  converted 
into  sulphuric  acid. 

By  increasing  the  number  of  crucibles  it  will  be  possible 
to  conduct  at  least  from  four  to  six  of  these  decompositions 
simultaneously,  and  by  using  a  volumetric  method  of  esti- 
mating the  sulphuric  acid,  a  sulphur  determination  can 
easily  be  executed  in  forty  minutes. 

Experience  has  demonstrated  that  0.1-0.2  gram  of 
material  will  require  about  20-25  grams  of  caustic  potash. 

Frankel  has  conclusively  demonstrated  that  the  arsenic 
contained  in  metallic  arsenides,  e.  g.,  arsenopyrite,  rammels- 
bergite,  etc.,  can  be  entirely  converted  into  arsenic  acid  by 
the  above  method.  He  recommends  conditions  analogous 
to  those  employed  with  the  sulphides. 

The  current  will  also  completely  decompose  the  mineral 
chromite.  For  a  quantity  of  material  varying  from  o.i- 
0.5  gram  use  from  30-40  grams  of  stick  potash  and  a  cru- 
cible slightly  larger  than  that  recommended  in  the  oxida- 
tion of  sulphides  and  arsenides.  The  current  should  not 
exceed  one  ampere.  Thirty  minutes  will  be  sufficient  for 
the  oxidation.  At  the  expiration  of  this  period  allow  the 
mass  to  cool,  take  up  in  water,  filter  off  from  the  iron  oxide, 
acidulate  the  filtrate  with  sulphuric  acid,  add  a  weighed 
quantity  of  ferrous  ammonium  sulphate,  and  determine 
the  excess  of  iron  with  a  standardized  bichromate  solution, 
using  potassium  ferricyanide  as  an  indicator.  Upon  oxi- 
dizing 0.4787  gram  of  chromite  by  the  above  process 
51.77  per  cent,  of  chromic  oxide  was  obtained,  while  a  sec- 
ond sample  of  the  same  mineral,  oxidized  by  the  Dittmar 
method,  gave  51.70  per  cent,  of  chromic  oxide.  If  the 
chromium  be  estimated  volumetrically,  the  chromium  con- 
tent in  a  chrome  ore  may  be  ascertained  in  less  than  an 
hour. 


COMBUSTION    OF    ORGANIC    COMPOUNDS.  3!9 

8.  THE  COMBUSTION  OF  ORGANIC 
COMPOUNDS. 

LITERATURE. — Carrasco,    R.    Ace.    d.    Lincei    (5),    14,    608;    Taylor 

Thesis  (Johns  Hopkins  University,  1905). 

• 

For  the  combustion  of  organic  bodies  Carrasco  employs 
an  ordinary  combustion  tube  in  which  there  is  heated  a  wire 
of  platinum-iridium.  An  atmosphere  of  oxygen  is  main- 
tained throughout  the  entire  experiment  which  usually  occu- 
pies not  more  than  fifteen  minutes.  The  device  of  Taylor 
in  its  simplest  form  is  seen  in  Fig.  41.  "  It  consists  of  a 
thin  glass  combustion  tube  A  closed  at  one  end,  300  mm.  in 
length  and  15  mm.  in  internal  diameter.  Through  the  rub- 
ber stopper  in  its  open  end  there  pass :  ( i )  the  porcelain 
tube  C ,  which  has  a  length  of  250  mm.  and  a  diameter  of 
6  mm. ;  (2)  the  glass  tube  K,  through  which  the  products  of 
combustion  enter  the  absorption  apparatus;  (3)  the  rather 
stout  platinum  wire,  which  extends  from  F  to  /.  The  por- 
celain tube  C  is  joined  outside  of  the  stopper,  by  means  of 
rubber  tubing,  to  the  branched  glass  tube  D.  The  latter  is 
provided  with  a  stopper,  G,  through  which  passes  the  plati- 
num wire  E,  which  extends  into  the  porcelain  tube  to  the 
point  H,  where  it  is  joined  to  a  smaller  platinum  wire.  The 
small  wire  has  a  length  of  about  1.75  meters  and  weighs, 
approximately,  2.5  grams.  It  extends  from  its  junction  with 
the  larger  wire  at  H,  through  the  porcelain  tube  to  the  inner 
end  of  the  latter  and  then  returns  on  the  outside,  in  a  series 
of  suspended  coils,  to  the  point  /,  where  it  joins  the  larger 
wire  F.  Thicker  wire  is  used  from  F  to  /  and  from  E  to  H 
in  order  to  avoid  any  overheating  of  the  rubber  stopper  by 
the  current.  The  roll  of  copper  wire  gauze  B,  about  60  mm. 
in  length,  is  inserted  between  the  end  of  the  porcelain  tube 
and  the  boat  containing  the  substance  to  be  burned. 


320 


ELECTRO-ANALYSIS. 


FlG-  4i.  "  The    coil    is    prepared    by    first 

heating  the  wire,  while  stretched 
slightly,  either  by  passing  it  through 
a  flame  or  by  connecting  its  ends 
with  electric  terminals  and  passing 
a  current  through  it.  The  danger 
of  the  former  method,  which  is  ob- 
viated by  the  latter,  is  that  the  wire 
will  have  its  resistance  changed  at 
some  one  spot  by  being  drawn  out 
there  through  uneven  heating.  This 
also  serves  the  purpose  of  straight- 
ening the  wire  and  removing  some 
of  the  temper,  making  it  easier  to 
wind.  It  is  then  wound  upon  a 
screw  thread  of  such  size  that  the 
coil  will  have  an  approximate  diame- 
ter of  9  mm.  During  the  winding 
the  tension  of  the  wire  should  be 
kept  as  nearly  constant  as  possible. 
After  all  the  wire  has  been  placed 
upon  the  thread  it  may  be  easily  re- 
moved by  turning  the  screw,  the 
wire  being  held  firmly  by  the  fingers. 
.From  this  method  an  even  coil 
should  result  which  is  ready  to  be 
placed  upon  the  porcelain  stem  for 
use.  After  the  wire  has  been  used 
for  a  few  combustions  it  loses  its 
temper  and  the  coil  can  then  be 
reformed  by  simply  winding  it 
around  a  glass  rod  of  the  proper 
diameter. 


COMBUSTION    OF    ORGANIC    COMPOUNDS.  321 

1  The  heavy  wire  from  /  to  F  is  sharpened  at  one  end  and 
with  a  pair  of  forceps  forced  through  the  rubber  stopper. 
By  regulating  its  length  in  the  combustion  tube  the  coils 
may  be  brought  so  near  the  end  that  all  the  moisture  will  be 
driven  over  and  yet  not  near  enough  to  burn  the  stopper. 
The  longer  wire  from  H  to  E,  forming  the  second  terminal, 
is  passed  through  the  stopper  in  the  branched  tube  D  at  G 
and  the  end  of  the  tube  filled  with  sealing-wax.  The  sec- 
ond end  of  the  branched  tube  is  slipped  over  the  end  of  the 
porcelain  tube  and  closed  with  thick  rubber  tubing  tied  with 
waxed  shoemaker's  thread. 

'  The  pure  oxygen  or  air  enters  the  apparatus  at  D  and 
while  passing  over  the  portion  of  the  small  wire  which  is 
within  the  porcelain  tube  has  its  temperature  raised  more  or 
less  according  to  the  rate  of  its  flow.  It  is,  therefore, 
already  hot  when  it  enters  the  tube  C ',  where  the  combustion 
is  to  be  effected.  The  completeness  of  the  combustion  is 
probably  due,  to  a  large  extent,  to  the  temperature  to  which 
the  oxygen  is  heated  before  it  comes  in  contact  with  the 
vapors  to  be  burned.  This  hot  oxygen  is  also  of  especial 
advantage  not  only  in  keeping  the  roll  of  copper  gauze  next 
to  the  porcelain  tube  thoroughly  oxidized  at  all  times,  but 
in  heating  the  roll  to  such  a  temperature  that  it  can  be  acted 
upon  readily  by  the  vapors  of  the  substance  to  be  burned. 
The  excess  of  oxygen  and  the  products  of  the  combustion 
of  the  substance  pass  together  over  the  heated  coils  on  the 
outside  of  the  porcelain  tube,  completing  the  burning  of  any 
unoxidized  material  coming  from  the  rear. 

"  The  coils  are  supported  by  unglazed  porcelain  tubes. 
They  are  very  durable  and  they  are  not  hygroscopic  to  an 
appreciable  degree. 

"  The  roll  of  copper  wire  gauze,  B,  while  not  absolutely 
necessary  has  some  advantage  because  much  less  care  is  re- 


322  ELECTRO-ANALYSIS. 

quired  in  the  management  of  the  combustion  with  it  than 
without  it.  If  the  substances  are  liquids,  or  if  they  readily 
yield  large  quantities  of  inflammable  vapors  when  heated, 
it  must  be  inserted  between  the  material  and  the  end  of  the 
porcelain  tube  through  which  the  oxygen  enters. 

'  The  combustion  is  conducted  in  the  following  manner : 
"  Having  placed,  in  the  positions  indicated  in  the  figure, 
the  boat  containing  the  material  and  the  roll  of  copper  wire 
gauze  (which,  in  the  beginning,  may  or  may  not  be  oxidized) 
and  having  joined  the  tube  K  to  the  usual  train  of  absorption 
apparatus,  a  slow  current  of  dry  and  purified  oxygen  is 
admitted  and  the  electric  circuit  is  closed  through  a  regulat- 
ing rheostat.  Starting  with  a  current  of  about  one  ampere 
the  flow  is  gradually  increased,  at  the  rate  of  0.2  ampere 
every  two  or  three  minutes,  until  the  coils  assume  a  bright 
red  color  or  until  3.6  amperes  are  reached.  While  the  coils 
are  being  heated  a  lamp  having  a  broad,  thin  flame  is 
brought  under  the  roll  of  copper  wire  gauze  and  raised 
gradually  until  the  blue  portion  of  the  flame  touches  the  glass 
tube  on  its  under  side.  The  substance  in  the  boat  is  then 
heated  with  the  same  lamp,  or  with  another  which  is  held  in 
the  hand.  The  rate  of  heating  and  the  flow  of  oxygen  are 
so  regulated  with  respect  to  each  other  that  at  least  one  half 
of  the  roll  of  wire  gauze  is  kept  in  the  oxidized  condition 
during  the  entire  combustion.  After  the  formation  of  vola- 
tile products  has  ceased,  the  reoxidation  of  the  copper  pro- 
gresses rapidly  and  the  oxygen  enters  the  rear  compartment, 
burning  any  residue  of  carbon  upon  the  boat  or  upon  the 
glass. 

"  Having  finished  the  combustion  of  the  substance,  the 
current  of  oxygen  is  replaced  by  one  of  dried  and  purified 
air,  and  the  flow  of  the  latter  continued  until  the  products  of 
the  combustion  have  all  been  expelled  from  the  space  behind 


COMBUSTION    OF    ORGANIC    COMPOUNDS.  323 

the  wire  gauze.  It  is  here  that  a  miscalculation  is  likely  to 
be  made.  The  time  required  for  the  complete  removal  of 
these  products  depends,  principally,  upon  the  freedom  of 
diffusion  through  the  gauze  and  for  this  reason  it  should 
not  be  rolled  too  tightly. 

"  The  apparatus,  already  described,  is  adapted  to  the  com- 
bustion of  those  solids  and  liquids  which  consist  of  carbon 
and  hydrogen,  or  of  carbon,  hydrogen  and  oxygen. 

'  The  heating  of  the  roll  of  wire  gauze  B,  and,  at  times,  of 
the  substance  also,  is  facilitated  by  inverting  over  the  tube, 
at  a  little  distance  above  it,  a  trough  of  asbestos  board,  the 
side  of  a  trough,  at  the  back,  being  much  deeper  than  in 
front.  This  arrangement  is  supported  in  its  position  by  a 
rod,  which  is  inserted  in  a  heavy  block,  resting  upon  the 
work  table  behind  the  tube.  The  device  is  also  of  advantage 
in  protecting  the  tube  from  draughts  of  cold  air  during  the 
combustion  and  during  the  subsequent  cooling  period.  The 
portion  of  the  glass  tube  which  is  occupied  by  the  porcelain 
tube  and  the  platinum  wire  is  protected,  on  the  bottom,  by 
a  semi-circular  strip  of  asbestos  board  which  is  inserted  in 
the  clamp  between  the  lower  jaw  and  the  glass.  To  protect 
the  upper  portion  of  the  tube  in  the  same  region,  a  semi- 
circular trough  of  mica  is  inverted  over  it,  behind  the  clamp, 
in  such  a  manner  that  the  lower  edges  of  the  mica  rest  in 
the  trough  below.  The  mica  is  made  to  keep  its  curved  form 
by  fastening  it  to  narrow  strips  of  metal  and  bending  the 
latter  to  the  required  shape. 

"  The  cooling  of  the  tube  requires  some  care.  The  cur- 
rent should  be  reduced  quite  gradually,  following  the  reverse 
of  the  heating  process,  and  it  is  well,  also,  as  soon  as  the 
combustion  is  finished,  to  cover  the  portions  of  the  glass 
tube  which  is  beyond  the  porcelain  one  with  the  soot  from  a 
smoky  flame  and  to  take  any  other  measures  for  the  protec- 


324  ELECTRO-ANALYSIS. 

tion  of  the  tube  which  will  contribute  toward  the  proper 
annealing  of  the  glass.  Care  must  likewise  be  taken  never 
to  allow  the  platinum  coils  to  come  in  contact  with  the  glass 
either  while  heating  or  cooling  the  tube,  since,  in  the  former 
case,  the  metal  is  likely  to  stick  to  the  glass,  while  in  the 
latter,  the  tube  is  quite  sure  to  crack  at  some  lower  temper- 
ature. Further,  the  coils,  after  being  used  for  some  time, 
show  a  tendency  to  increase  in  size  towards  the  end  of  the 
porcelain  tube,  and,  if  they  approach  too  nearly  the  inner 
diameter  of  the  combustion  tube,  the  wire  must  be  taken  out 
and  rewound.  The  difficulty  of  keeping  the  coils  away  from 
the  glass  while  they  were  hot,  led  to  the  placing  upon  the 
inner  end  of  the  porcelain  tube  of  a  small  platinum  disk. 
The  porcelain  tube  was  ground  down  at  the  end  until  it  was 
practically  square  and  the  disk,  which  was  a  little  smaller 
than  the  internal  diameter  of  the  combustion  tube,  was  fitted 
eccentrically  upon  it  so  that  the  coils  were  held  the  same 
distance  from  the  glass  tube  at  all  points.  Small  holes  were 
drilled  in  the  disk  to  allow  the  free  passage  of  the  vapors. 
As  the  small  wire  of  the  coils  only  comes  in  contact  with 
the  platinum  disk  at  one  point  it  does  not  heat  the  latter  hot 
enough  to  affect  the  glass  tube  injuriously.  The  porcelain 
tube  and  coils  are  thus  always  kept  in  the  same  relative  posi- 
tion to  the  glass  tube  while  the  combustion  is  not  in  any  way 
interfered  with.  With  the  proper  care  a  good  piece  of 
glass  tubing  can  be  used  for  a  large  number  of  combustions. 
"  The  time  required  for  a  combustion  does  not,  ordinarily, 
exceed  half  an  hour,  and  it  may  be  reduced  to  twenty 
minutes,  or  even  less,  if  the  substance  to  be  burned  is  of  such 
a  character  that  the  roll  of  wire  gauze  can  be  dispensed  with. 
Its  omission  is  not,  however,  recommended  at  any  time, 
except  to  those  who  have  had  some  experience  with  the 
method. 


COMBUSTION    OF    ORGANIC    COMPOUNDS. 


325 


"  At  the  highest  temperature  employed 
during  the  combustion  (at  a  bright  red, 
but  not  a  white  heat),  especially  when  the 
wire  is  new,  there  is  a  sensible  volatiliza- 
tion of  the  platinum.  This  volatilization 
of  platinum  in  an  atmosphere  of  oxygen,' 
even  at  comparatively  moderate  tempera- 
tures, has  been  repeatedly  noticed  by  others. 
The  volatilized  metal  settles  upon  the  sur- 
face of  the  glass  and  porcelain  tubes  as  i 
dark  deposit,  which,  at  first,  may  be  mis- 
taken for  carbon.  The  presence  of  such 
films  of  volatilized  platinum  upon  the  in- 
ner surface  of  the  tube  is,  of  course,  by 
its  catalytic  action,  of  some  assistance  in 
the  combustion. 

"  The  objections  to  and  difficulties  in 
the  use  of  the  short,  closed  combustion 
tube  represented  in  Fig.  41  are  wholly  ob- 
viated by  using  a  somewhat  longer  tube 
which  is  open  at  both  ends,  as  represented 
in  Fig.  42.  In  this  arrangement  the  boat 
is  introduced  from  the  rear  and  there  is 
placed  behind  it  a  second  roll  of  copper 
wire  gauze,  about  60  mm.  in  length.  The 
stopper  in  the  front  end  of  the  combustion 
tube,  the  forward  roll  of  copper  wire  gauze 
and  also  the  apparatus  as  a  whole,  are 
never  disturbed.  Each  roll  of  wire  gauze 
is  heated  by  a  lamp  giving  a  broad,  thin 
flame  and  there  is  inverted  over  both  rolls 
and  the  space  between  them  the  asbes- 
tos shield  already  described.  The  lamps 


FIG.  42. 


326  ELECTRO-ANALYSIS. 

should  be  raised  until  the  bottom  of  the  tube  is  just  within 
the  blue  region  of  the  flames.  To  prevent  any  sagging  of 
the  combustion  tube  while  hot,  it  is  supported  at  a  point 
beneath  the  end  of  the  porcelain  tube  by  a  forked  or  notched 
standard,  which  is  placed  under  the  asbestos  trough  in  which 
the  front  portion  of  the  apparatus  lies. 

'  The  combustion  is  conducted  in  the  same  manner  as  in 
the  short,  closed  tube,  except  that  a  slow  current  of  oxygen 
or  air  is  admitted  from  the  rear  during  the  entire  experi- 
ment. This  prevents  any  accumulation  of  volatilized  mat- 
ter in  the  back  part  of  the  tube  and  aids  in  the  expulsion  of 
the  products  of  combustion  from  the  space  occupied  by  the 
boat. 

"  If  the  substance  to  be  burned  is  very  volatile,  it  is  ad- 
visable to  introduce  air  and  not  oxygen  in  the  rear,  and  to 
employ,  behind  the  boat,  a  roll  of  gauze  which  is  only  par- 
tially oxidized.  In  this  way  the  vapors  of  the  substance 
may  be  diluted  with  nitrogen  to  any  desired  extent. 

"  With  this  apparatus  a  Marchand  tube,  filled  with  calcium 
chloride,  is  used  to  absorb  the  water  vapors  formed,  because 
the  end  of  the  tube  can  be  placed  directly  in  the  stopper  of 
the  combustion  tube,  thus  doing  away  with  the  connection 
tube  K.  No  trouble  is  experienced  with  this  arrangement  in 
getting  the  water  vapor  ready  to  weigh  by  the  time  the  com- 
bustion is  completed.  When  the  Marchand  tube  is  re- 
moved from  the  absorption  train  its  ends  are  closed  by  small 
pieces  of  rubber  tubing  carrying  glass  plugs. 

"  The  clamp  at  the  rear  is  required  only  as  a  support  and 
it  should  not  grip  the  tube  so  tightly  as  to  prevent  the  free 
movement  of  the  latter,  back  and  forth  through  the  former. 

"  In  the  following  determinations  of  carbon  and  hydrogen 
in  cane-sugar,  which  were  made  for  the  purpose  of  testing 
the  method,  the  short,  closed  tube  was  employed  and  the 


COMBUSTION    OF    ORGANIC    COMPOUNDS. 


32; 


roll  of  wire  gauze  was  omitted.  A  clay  tobacco  pipe  stem 
served  for  the  introduction  of  oxygen  and  the  effect  of  its 
use  is  evident  in  the  high  percentages  of  hydrogen  which 
were  obtained  in  the  first  four  analyses.  In  the  last  two 
analyses,  in  which  normal  quantities  of  hydrogen  were 
obtained,  the  pipe  stem  was  thoroughly  burned  out  in  a 
current  of  oxygen  before  beginning  the  combustion : 


WEIGHT  OF 
SUGAR.     GRAM. 

CARBON  FOUND. 
PER  CENT. 

HYDROGEN 
FOUND.    PER  CENT 

TIME  OCCUPIED  IN 
COMBUSTION. 
MINUTES. 

0.1364 

41-95 

6.86 

25 

O.II88 

42.03 

6.63 

18 

o.  1227 

42.03 

6.65 

18 

0.1382 

42.07 

6-73 

18 

O.II54 

42.  II 

6.47 

18 

o.  2809 

42.03 

6.46 

45 

Theoretical,  42.09 

6-47 

"  The  current  at  the  highest  temperature  was  2.6  amperes 
at  48  volts.  In  these  combustions  a  coil  of  No.  32  wire 
(B.  &  S.  gauge)  was  used,  but,  as  is  stated  later,  it  was 
found  advisable  to  exchange  this,  in  the  combustions  of 
naphthalene,  for  a  greater  length  of  larger  wire. 

"  Careful  management  is  required,  even  in  the  combustion 
of  such  substances  as  sugar,  when  the  roll  of  wire  gauze  is 
omitted.  On  several  occasions,  when  it  was  attempted  to 
reduce  the  time  consumed  in  combustion  to  fifteen  minutes 
or  less,  small  explosions  occurred.  To  avoid  the  explosions, 
which  always  resulted  in  unburned  material  escaping,  the 
combustion  tube  was  lengthened  slightly  and  the  previously 
mentioned  roll  of  wire  gauze  was  inserted  between  the  boat 
and  the  end  of  the  porcelain  tube.  Combustions  of  toluene 
and  two  of  naphthalene  were  made  with  the  modified  ap- 
paratus with  the  following  results  : 


328 


ELECTRO-ANALYSIS. 

TOLUENE. 


WEIGHT  OF 

CARBON  FOUND 

HYDROGEN  FOUND. 

TIME  OCCUPIED  IN 

SUBSTANCE.   GRAM. 

PER  CENT 

PER  LENT. 

COMBUSTION. 
MINUTES. 

0.1057 

90.91 

8.62 

35 

0.0650 

91.25 

8.80 

35 

Theoretical,  91.24 

8.76 

NAPHTHALENE. 


WEIGHT  OF 
SUBSTANCE.  GRAM. 

CARBON  FOUND. 
PER  CENT. 

HYDROGEN  FOUND. 
PER  CENT. 

TIME  OCCUPIED  IN 
COMBUSTION. 
MINUTES. 

o  1184 

0.1252 

93-54 
93-49 
Theoretical,  93.70 

6.36 
6-39 

6.36 

55 
55 

The  Combustion  of  Substances   Containing  Nitrogen. 

"  For  the  determination  of  carbon  and  hydrogen  in  com- 
pounds containing  nitrogen,  there  are  placed  in  the  combus- 
tion tube:  (i)  a  roll,  100  mm.  in  length,  of  wire  copper 
gauze  which  has  been  reduced  in  the  usual  way  by  methyl 
alcohol;  (2)  a  roll,  80  mm.  in  length,  of  wire  gauze  which 
has  been  well  oxidized;  (3)  the  boat  containing  the  sub- 
stance; (4)  a  short  roll  of  wire  gauze  also  well  oxidized. 

"  During  the  combustion  each  of  the  three  rolls  is  heated 
by  a  burner  giving  a  broad,  thin  flame,  the  last  lamp  serving 
also  for  heating  the  substance.  The  portion  of  the  tube 
occupied  by  the  copper  is  covered  with  a  screen  of  asbestos 
board,  to  insure  a  sufficiently  high  temperature  for  the  re- 
duction of  the  nitric  oxide.  The  flow  of  the  oxygen  through 
the  porcelain  tube  is  so  regulated  that  only  about  one-quarter 
of  the  copper  roll  (i)  is  oxidized,  while  at  the  rear  it  is 
admitted  as  rapidly  as  may  be  necessary  to  keep  a  portion 
of  the  second  roll  (2)  at  all  times  in  an  oxidized  condition. 


COMBUSTION    OF    ORGANIC    COMPOUNDS.  329 

The  Combustion  of  Halogen  Compounds. 

'  To  prepare  the  apparatus  for  the  analysis  of  substances 
containing  the  halogens,  a  piece  of  silver  foil,  about  50  mm. 
in  width,  is  rolled  up  with  a  sheet  of  thick  paper,  which  is 
afterwards  withdrawn.  The  silver  roll  is  placed  in  the  tube 
quite  close  to  the  end  of  the  porcelain  tube  and  is  not  directly 
heated  during  the  combustion.  In  other  respects  the  arrange- 
ments are  the  same  as  for  the  combustion  of  non-nitrogenous 
compounds.  A  roll  of  well-oxidized  copper  wire  gauze  fol- 
lows the  one  of  silver,  then  the  boat  containing  the  sub- 
stance and,  finally,  a  second  roll  of  oxidized  copper  wire 
gauze. 

"  During  the  combustion  there  is  formed  a  quantity  of 
fusible  cuprous-halogen  salt,  which  deposits  itself,  more  or 
less,  upon  the  inner  surface  of  the  glass  tube,  but  does  not, 
at  any  time,  get  beyond  the  silver  foil  into  the  space  occu- 
pied by  the  porcelain  tube  and  platinum  wire.  On  cooling, 
the  cuprous-halogen  salt,  in  accordance  with  the  well-known 
behavior  of  such  compounds,  absorbs  large  quantities  of 
oxygen,  only  to  give  it  up  again  when  the  apparatus  is 
reheated  in  a  succeeding  experiment.  At  the  same  time 
the  copper  wire,  in  the  oxidized  rolls,  grows  thinner  and  be- 
comes quite  brittle. 

"  The  quantity  of  cuprous  salt  accumulates,  after  a  few 
combustions,  to  such  an  extent  that  the  time  required  for 
its  oxidation  is  considerable.  Hence,  it  is  well  frequently 
to  cleanse  the  combustion  tube  and  to  renew,  at  the  same 
time,  the  oxidized  rolls  of  copper  wire  gauze. 

The  Combustion  of  Sulphur  Compounds. 

"  The   determination   of   carbon   and  hydrogen   in   com- 
pounds containing  sulphur  presents  no  difficulty.     The  only 
29 


33O  ELECTRO-ANALYSIS. 

change  which  it  is  necessary  to  make  in  the  simple  arrange- 
ment for  non-nitrogenous  and  non-halogen  compounds,  in 
order  to  adapt  the  method  to  the  combustion  of  sulphur 
compounds,  is  to  substitute  lead  chromate  for  the  roll  of 
oxidized  copper  wire  gauze  which  is  nearest  the  end  of 
the  porcelain  tube.  Instead  of  maintaining  the  lead  chro- 
mate in  its  position  in  the  tube  by  means  of  plugs  of  asbestos 
or  of  wire  gauze,  it  has  been  found  more  convenient  and 
better  for  the  glass  tube  to  introduce  it  in  the  form  of  a 
cartridge.  This  is  prepared  by  filling,  with  the  loose,  granu- 
lar "chromate,  a  shell  made  from  very  fine  copper  wire 
gauze." 


INDEX. 


Accumulator,  2 
Ammeters,  9,  u,  17 
Ampere,  7 
Amperemeter,  i,  9 
Anions,  I 

determination   of,   296 
Anode,   i,   n 
dish,  73 
spiral,  73 
Antimony,   determination  of,   171- 

177 
rapid    precipitation    of,     177- 

179 

separation  from  arsenic,  251 
bismuth,  225 
copper,   183,   184 
lead,    233 
mercury,  215 
silver,  238 
tin,  251-255 

Arsenic,  determination  of,   180 
oxidation  of,  318 
separation  from  antimony,  251 
bismuth,p  225 
cadmium,  205 
copper,   184,   185,   186 
lead,  234 
mercury,  215 
silver,    238 
tin,   255 

Barium,    determination    of,    307 
separation    from   calcium   and 

magnesium,   309,   310 
separation    from    iron,    311 
separation    from    magnesium, 

3ii 

separation  from  uranium,  312 
Battery,  Bunsen,  10 

storage,  2,  13 

Bismuth,    determination   of,   95-98 
rapid   precipitation   of,   98,   99 
rapid  precipitation   with  mer- 
cury  cathode,   99-100. 


Bismuth,  separation  from  alumin- 
ium, 225 

antimony,  225 

arsenic,  225 

barium,  225 

cadmium,  225 

calcium,  226 

chromium,  226,  227 

cobalt,  227 

copper,  227 

gold,  228 

iron,  228,  229 

lead,  229,  230 

magnesium,  230 

manganese,   230 

mercury,   231 

molybdenum,  231 

nickel,  231 

palladium    and    platinum, 
231 

potassium,  231 

selenium^  231 

silver,  231 

sodium,  232 

strontium,   232 

tellurium,   232 

tin,  232 

tungsten,  232 

uranium,  232 

vanadium,  232 

zinc,  233 
Board,  distributing,   12 

switch,    12 

Bromine,    separation    from    chlor- 
ine, 289 
Bunsen  cell,  10 

Cadmium,  determination  of,  81-84 
rapid  precipitation  oi,  84-88 
rapid    precipitation    of,    with 

mercury  cathode,  8^-89 
separation     from     aluminium, 

203,  204 
antimony,  205 
arsenic,  205 


331 


332 


INDEX 


Cadmium,  separation  from  barium, 
strontium,  etc.,  205 

beryllium,   205 

bismuth,  205 

chromium,  205 

cobalt,  205 

copper,  186,  187,  188,  206 

gold,  206 

iron,  207 

lead,  207 

magnesium,   208 

manganese,  208,  209 

mercury,  209 

molybdenum,  209 

nickel,  209,  210 

osmium,  210,  211 

selenium,  211 

silver,  211 

sodium,  211 

strontium,    211 

tellurium,  211 

tin,  211 

tungsten,  211 

uranium,  211 

vanadium,  211 

zinc,  211,  212,  213,  214 
Cations,  I 

determination  of,  296 
Cathode,  i 

mercury,   55 

Chromite,  oxidation  of,  318 
Chromium,  determination  of  144- 

145 

rapid  precipitation   with   mer- 
cury cathode,  145,   146 

separation     from     aluminium, 

273 

beryllium,  274 

Cobalt,   determination  of,   122-126 
rapid    precipitation    of,     130- 

.133 

with     mercury     cath- 
ode, 133 

separation   from  bismuth,  227 
cadmium,  206 
copper,   189,  190 
iron,  262 
manganese,  267 
mercury,  218 
nickel,  267 
silver,  236 
zinc,  268 

Combustion      of      organic      com- 
pounds, 319-330 


Copper,  determination  of,  63-72 
rapid   precipitation   of,   72-77 
with     mercury     cath- 
ode, 77-80 

separation     from     aluminium, 
181,   182,  183 

antimony,  183,  184 

arsenic,   184,   185,   186 

barium,    strontium,    mag- 
nesium, etc.,  185. 

bismuth,    186 

cadmium,   186,    187,   188 

calcium,    188 

chromium,   188 

cobalt,   189,   190 

gold,  190 

iron,  190,  191,  192,  193 

lead,    193,    194 

magnesium,   194 

manganese,  194,  195 

mercury,  196 

molybdenum,   196 

nickel,  196,  197,  198 

palladium,   198 

platinum,   198 

potassium,   198 

selenium,  198,   199 

silver,   199 

sodium,  199 

strontium,   199 

tellurium,    199,   200 

thallium,  200 

tin,  200 

tungsten,"  200 

uranium,  200,  201 

vanadium,  201 

zinc,  20 1,  202,  203 
Current,  action  upon  compounds,  I 
density,    10 
electric  light,  3 
measuring  of,  9 
reduction  of,  5,  7 
separations,  39 

Decomposition   pressure,   32,   33 
Determination  of  metals,  63 
Distributing  board,  14 
Dynamos,  2 

Electric  current,   sources  of,  2 

light  current,  3 

motor,  96 
Flectro-analysis,   I 
Electro-chemical  laboratory,   12 


INDEX 


333 


Electrode,  auxiliary,  279 
Electrolysis,  defined,  i 
Electrolyte,    I 

Galvanometer,  9 

sine,  9 

tangent  9 
Gold,  determination  of,  162,   164 

rapid     precipitation     of,     164, 

165 

with     mercury     cath- 
ode,  165 

separation  from  antimony,  246 
arsenic,  250 
cadmium,  246,  247 
cobalt,   247 
copper,  247 
iron,  248 

molybdenum,  249,  250 
nickel,  248 
osmium,  249 
palladium,  248 
platinum,  249 
tungsten,  249,  250 
zinc,  249 

Halogen    compounds,    combustion 

of,  329 
Halogens,    determination   of,   285 

separation  of,  287 
Historical  account,  19-31 

Indium,  determination  of,  150,  151 
Iodine,  determination  of,  286 

separation   from  bromine,  289 

chlorine,  288 
Ions,  33 
Iron,  determination  of,  138-142 

rapid     precipitation     of,     142, 

143 
with      mercury      cathode, 

1.43,  144 
separation     from     aluminium, 

256,  257,  259 
beryllium,  ?s8 
bismuth,  228,  229 
cadmium,  207 
cerium,  261 
chromium,  262 
cobalt,  262 
copper,  190,  191,  192 
lanthanum,  260 
lead,  234 
manganese,  262,  263,  264 


Iron,    separation    from    mercury, 

219 

neodymium,   261 
nickel,  264,  265,  266 
phosphoric   acids,    266 
praseodymium,  260 
silver,  243 
thorium,   260 
titanium,  261,  266 
uranium,  259,  266 
vanadium,  258 
yttrium,  261 
zinc,  266,  267 
zirconium,  261 

Laboratory,    electrochemical,    12 
Lead,  determination  of,  100-103 
rapid  precipitation  of,  103-104 
separation  from  alkali  metals, 
barium,  beryllium,  cad- 
mium,   calcium,    cobalt, 
iron,  magnesium,  nickel, 
uranium,    zinc,    zircon- 
ium,  234 

aluminium,  233 

antimony,   233 

arsenic,  234 

bismuth,  235 

copper,  235 

gold,  235 

manganese,  235,  236 

mercury,  236 

selenium,  236 

silver,  236,  237 

tellurium,  237 

tin,  237 

Magneto-machines,   2 

Manganese,  determination  of,  134- 

138 

rapid  precipitation   of,    138 
separation     from     aluminium, 

.134 

bismuth,  230 

cadmium,  208,  209 

cobalt,  267 

copper,  194,  195 

iron,  262,  263,  264 

mercury,   220 

nickel,  268 

zinc,  269 

Measuring  currents,  9 
Mercury,   determination   of,  89-93 
rapid  precipitation  of,  93-94 


334 


INDEX 


Mercury,   rapid  precipitation  with 

mercury   cathode,  94-95 
separation     from     aluminium, 
214,  215 

antimony,  215 

arsenic,  215,  216 

barium,     strontium,     etc., 
216 

bismuth,  216,  217 

cadmium,  217 

calcium,  218 

chromium,  218 

cobalt,  218 

copper,  218,  219 

gold,  219 

iron,  219,  220 

lead,   220 

magnesium,  220 

manganese,  220 

molybdenum,  221 

nickel,  221 

osmium,  221 

palladium,  221 

platinum,  221 

potassium,   222 

selenium,   222 

silver,  222 

sodium,  222 

strontium,  222 

tellurium,  222 

tin,  222,  223 

tungsten,  223 

uranium,  223,  224 

vanadium,  224 

zinc,  224 
Metals,  separation  of,  181,  274 

additional    remarks,    274 
Milliamperemeter,   9 
Molybdenum,     determination     of, 

157,  161 
rapid   precipitation    of,    161 

with     mercury     cath- 
ode, 162 
separation  from  cadmium,  209 

mercury,  221 

silver,  244 

vanadium,  272 

Nickel,   determination  of,   122-126 
rapid  precipitation  of,  126-129 
with     mercury     cath- 
ode,   129,    130 

separation     from     aluminium, 
264 


Nickel,   separation    from   bismuth, 
231 

cadmium,   209,  210 

cobalt,   267 

copper,  196,  197 

iron,  264,  265 

lead,  234 

manganese,  268 

mercury,  221 

silver,  244 

zinc,  268,  269 

Nitric  acid,  determination   of,  289 
rapid    determination    of,    290- 

296 
Normal  density  denned,   10 

Organic  compounds,  combustion 
of,  319-330 

Osmium,   181 

Oxidations  by  means  of  the  cur- 
rent, 314 

Palladium,    determination    of,    153 
rapid     precipitation     of,     154- 

156 
separation  from  iridium,  250 

mercury,  221,  250 
Phosphoric    acid,    separation,    etc., 

266 

Platinum,    determination    of,    151 
rapid  precipitation  of,   152 
metals-,  250 

separation  of,  250 
separation    from   iridium,    250 
Pole  pressure,   n 
Potassium     ferricyanide,     analysis 

of,  306 

ferrocyanide,   analysis   of,  306 
separation   from   calcium   and 

magnesium,    309 
.    iron,  311 

sulphocyanide,  analysis  of,  300 
Potential  across  the  poles,   n 
Precipitation   of  metals,  rapid,  41 

Resistance    coils    and    frames,    6, 

7,  8 

Rheostat,  6,  7,   17,  281 
Rhodium,    determination    of,    156, 

250 

rapid     precipitation     of,     156, 
157 


INDEX 


335 


Rotating  anode,  42 

and  mercury  cathode,  58, 

296 
cathode,  46,  49,  51 

Separation,    constant    current,    39, 

4i 

Separation  of  metals,   181,  274 
Silver,    determination    of,    104-107 
rapid  precipitation  of,  107-108 
with     mercury     cath- 
ode,  1 08 

separation     from     aluminium, 
237,  238 

antimony,  238 

arsenic,  238 

barium,  239 

bismuth,  231,  239 

cadmium,  239 

calcium,  239 

chromium,  239 

cobalt,  239,  240 

copper,  240,  241,  242,  243 

gold,  243 

iron,  243 

lead,   236,   243 

lithium,  243 

magnesium,  243 

manganese,  243 

mercury,  244 

molybdenum,  244 

nickel,   244 

osmium,  244 

palladium,  244 

platinum,  244 
-  -  potassium,    244 

selenium,  245 

tellurium,  245 

tin,  245,  246 

tungsten,  244 

uranium,  246 

zinc,  246 
Sodium  bromide,  analysis  of,  300 

305 

carbonate,  analysis  of,  305 
chloride,  analysis   of,  294 
iodide,  analysis  of,  300 
separation    from   calcium   and 

magnesium,  308 
iron,  311 
uranium,   312 
sulphide,  analysis  of,  313 


Storage   cells,   2,    13 
Strontium,    determination   of,   307 
separation    from   calcium   and 

magnesium,  310 
iron,  311 
magnesium,  311 
Sulphur    compounds,    combustion 

of,  329 

Sulphur,  oxidation  of,  314 
Switchboard,   14 

Table,  working,  18 

Tangent  galvanometer,  9 

Tellurium,   179,   180 

Thallium,    determination    of,    149, 

150 

Theoretical    considerations,   32 
Thermopile,  2 

Tin,  determination  of,   166-168 
rapid    precipitation     of,     168- 

170 

with     mercury     cath- 
ode,  170-171 
separation      from      antimony, 

251-255 

arsenic,  255 

bismuth,  232 

cadmium,  211 

copper,  200 

lead,  237 

manganese,  255 

mercury,  222 
Trisodium  phosphate,  analysis  of, 

306 
Tungsten,  41,  180 

Uranium,    determination    of,    146- 

148 

rapid  precipitation  of,  149 
separation    from   barium,   270, 

271 

calcium,  271 
magnesium,  271 
zinc,  272 
Vanadium,    180 
Voltage,   ii 
Voltameter,  9 
Voltmeter,  11,  64 

\Vorking  table,  18 

Zinc,   determination  of,   109-116 
rapid     precipitation     of,     116- 
120 


136 


INDEX 


Zinc,  rapid  precipitation  with  mer- 
cury cathode,  120-122 
separation     from     aluminium, 

270 

bismuth,   233 
cadmium,   211-214 
copper,  201-203 


Zinc,    separation    from    iron,    266, 

267 

lead,  234 

manganese,  269,  270 
mercury,  224 
silver,  246 


THE 
UNIVERSITY 


\ 


[S  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


:NITIAL  FINE  OF  25  CENTS 

BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1..po'VON  THE  SEVENTH  DAY 
OVERDUE. 


i'*MAY^14  ' 


1907 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


